1. Like spraying cats, first eliminate any medical issue.
He may have a UTI so I'd take him for a vet visit.
I did find this short article and didn't even think about the neutering issue:
Spraying (especially by males) may mean a number of things, and many rabbits are surrendered to shelters or set loose because their owners don't know what their rabbit is saying to them. The biggest reason rabbits spray, especially males, is hormonal - when they begin to mature, the hormones will kick in and they will start spraying as a response to puberty. This is why you should get your bunny spay or neuter your bunny as soon as he or she is old enough. Bunnies will also spray to claim territory, although this behavior should also be ameliorated by getting your bunny fixed. Believe it or not, when you get sprayed by a rabbit, it is actually a show of affection. The rabbit is claiming you as his property and marking you with his scent. Most of the time this behavior is easily cured after spaying/neutering, although sometimes more training is required.
If your rabbit is litterbox trained and sometimes leaves puddles elsewhere (especially just outside the litterbox), he may be complaining about something. Rabbits don't just forget their litterbox habits - this could be an indication that he isn't feeling well. If it appears that your rabbit is straining to urinate, take him to the vet - it may be a sign of calcium buildup in the bladder (sludge or a stone). See also Health. Male rabbits that have not been neutered will also mark female rabbits as their territory by spraying them, and sometimes female rabbits spray as well.
Sometimes even a well-trained rabbit will spray in response to external stresses, such as the introduction to a new pet or person the rabbit doesn't know yet. This is typically a temporary situation and will abate once the rabbit is more comfortable with its surroundings.