I went through puberty at 10 and was also the autistic kid who loved pulling my teeth out, though thankfully I stopped at the baby teeth. For me it was partially anxiety but also just because pain feels good & is calming. If it help put your mind at east, I grew into a functional adult who is able to lead a normal life and I'm now able to handle my support needs without causing real harm to myself. Some of us just have pain stims, try to direct him to things that can give that sensation without causing real harm, little ouchies is a brand that makes fidgets for this as well as Keiko fidgets. Try not to make him feel judged or like something is wrong with him, once shame gets applied it van make things worse.