Dogs just poop where ever they're allowed to when they need to poop, but pee is what they mark with. Try getting some of his tinkle on a paper towel or newspaper or something like that and putting that in the spot in the yard you want him to use as his toilet place. Is there the scent of another dog in that place that he might be picking up on? Yoshi was a little intimidated to do his business in the yard (either pee or poo) at first I think because Boo's bathroom scent was still pretty well established in the yard. I think it was the sent of his own pee on a paper towel that I put in the spot I wanted him to use in the yard that finally encouraged him to use that spot. I also had to make sure that he didn't pick up on my watching him in the yard because that just made him feel like he was doing something wrong and wouldn't go. I finally figured out that just my presence was putting out some vibe like that to him, so I forced myself to stay inside and secretly peek at what he was doing through the window.
Try using his pee scent instead of his poop scent to mark his place for him and see if that makes a difference. Also take yourself out of the yard so he won't feel like he's being watched when you want him to go to the bathroom in the yard and thus be too nervous by your presence to go to the bathroom. Is there a way you can secretly watch him in the yard to see what he's doing (or not doing) out there? And as soon as he does finally do any of his business in the yard (even if it's not exactly the right spot) immediately praise him well so he gets the idea that you want him to go in the yard. The more he does go in the yard the more his own scent will take over any other dog's scent that might have been intimidating him into not wanting to go potty there. I still praise Yoshi whenever I notice he goes in the potty place in the yard just for positive reinforcement.
As for his accidents in the house... you have to crate him. Once he learns that he can go in the house and not get in trouble for it you have to start all over with potty training. Don't feel bad about the crate. He'll actually be more comfortable and feel more safe in it when he's alone in the house. He has to learn NOW that going in the house is not allowed or re-training will just be more difficult and take longer. Since he IS potty trained he just needs to be re-trained for THIS home.
Yoshi was the same way when he first came to live with me... he was already potty trained, but he had to learn right away that my house had the same no-potty-in-the-house rule that he learned where he lived before. That only took a couple of days for him to figure out since any time he was not in the crate I was right there watching him and could immediately catch him in the act and correct him. It was only twice that he tried to potty in the house and I corrected him that he understood he wasn't allowed to go potty in my house either just like where he lived before. But if you can't catch them in the act and correct him all he's learned is that he CAN potty in the house and not get in trouble for it. You're going to have to crate him until he figures out where he can go and where he can't, but since he's already potty trained this re-training shouldn't take long at all (no where near as long as their first learning when they're babies).
Try his pee scent in the yard, and the more of it, the better. I think it makes a lot of difference to a dog to have their pee scent rather than their poop scent because it's pee that they mark with, not poop. It was putting Yoshi's own pee scent in the potty place in the yard plus my not letting him know I was spying on him in the yard that worked for him. And probably his learning right away that he wasn't allowed to go potty in the house.
I wonder about using that spot as his potty place though. Last spring I planted grass seed again, and Yoshi seemed put off by the disturbed dirt. It could be that this dog just doesn't feel comfy with the spot you chose because of the planting. I've noticed that though he loves to dig up the yard all over the one place Yoshi absolutely won't dig in is his potty spot, and he doesn't like it if the earth has been disturbed in that spot. I never did plant any grass seed there because of it - he was very visibly put off by having the dirt in his potty spot disturbed. Now that I think of it, he never seems to want to do his business in a place where the dirt has recently been disturbed. When we went out for our walks there was one little grassy area he always liked to pee in, and one time they were doing some kind of road repair there, and the big machines used that area to back up in getting it all dug up and rutted, and Yoshi didn't want to pee there again until months later when it finally smoothed out again and grew new grass. Maybe there's just something dogs don't seem to like about going in a place where the dirt has been recently disturbed. Funny, but he just didn't like it - didn't even want to walk around that area and sniff.
I also learned the hard way both this year and last year that grass seed and new grass don't like urine. And it doesn't take much urine at all to kill new grass right down to the roots. If you want new grass to grow in that spot, you don't want him using that spot to pee for a good year when the grass is thick and firmly established. Dog pee kills even well established grass and shrubbery if it keeps getting peed on regularly.
Ok, so try using his pee scent instead of his poop scent, maybe try a different spot in the yard where the dirt isn't disturbed in case he's just put off by going potty on disturbed dirt, don't let him know you watch what he's doing in the yard so he doesn't feel nervous about what he's doing or not doing out there, and crate him so he knows he can't use the house as a place to go potty (since he's already potty trained, this shouldn't take long at all). Also keep him blocked off from that room he's done his business in before so he can't go back to where he's left his potty scent, and use a neutralizer on that area to kill the scent. And when he does finally use the yard even if it's not exactly the spot you'd like him to use, praise him well immediately so he learns that you want him to do his business in the yard and worry about exactly where in the yard you'd prefer him to go potty later (some dogs are just fussy and want to choose their own spot though they can be trained to use a certain spot once they get it all figured out they they have to potty in the yard and not the house).
If you do all this he should have it figured out in no time at all and get to have the run of the house again when you aren't home. It only took a couple of days to re-train Yoshi about no going to the bathroom in the house because he was already potty trained and just needed to learn that the same no-potty-in-the-house rule applied to my house just as much as his old house.
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