From the descriptions of him, it sounds like he was a disturbed loner who never joined anything. He wasn't a "gun nut," as far as we know (that is, someone who collected or was especially interested in guns and shooting), although his mother might have been.
4. Telegraph reported both mother and father were Republican.
And several other reports sail the Newton area is pretty conservative. Quite possibly the mother belonged to NRA, if she got gun-training through them, or if membership was required at local gun-range.
Unknown if son was a member. Son didn't own any firearms, he took them from mother.