I've experienced kids biting, but they did not have SPD, so I'm not sure it's definitely connected to the SPD except that I've read that most kids with SPD are developmentally about one-third of their chronological age behind, so if you're son is 6, that would make him 4 developmentally, and I have heard of 4 year olds biting when they're stressed. The book, Effective Parenting for the Hard to Manage Child may supply with some helpful info or guidance.
I've posted many times that the Feingold Diet helped my daughter with behavioral outbursts, meltdowns and anger, so I can't repeat enough that I think everyone should try it. If the biting is related to frustration or anger, I think it would disappear on the diet. It's not easy to begin, but after 3-4 weeks you get used to it and after a couple of months it becomes a normal way for your family to eat. It is worth all the hours of shopping to comply with the diet to see my daughter wake up happy and singing every morning, going from 5-6 meltdowns a day to 5 in the last six months, and even those were related to giving her a food not allowed on the diet.
I encourage you to stick with this forum but be realistic that not every post will get replies. I've posted things that have not been responded to for a long time or ever. But the support and encouragement I've gotten since I joined have gotten me through some pretty low times. Please be patient with us. We're all dealing with children whose needs are very demanding and difficult. Sometimes I only make it here once a week (if that) to catch up.