you should maybe look at speaking to a integrated medicine specialist. especially some-one that works with nutrition. Not everyone is put on a diet, only if there is health issue to look at will they normally recommend cutting out certain foods because of certain elements in it.

You can ask if goat milk is ok for him or not. Also cashew and almond milk may works.

Hi RachelKa, gluten free diet is ideal, I would stress probiotics, and Vit.B(all, complex). As for dairy, my son drinks Lactaid, seems to be fine for his system.
I don't know what's trying to be accomplished here but I would suggest early intervention, speech therapy, behavior therapy, in addition to working with your son at home. My son started verbal, lost his speech, started speaking again. Now i don't promise you'll have the same success, because every child is different. Hard work is necessary to help our children, and never give up hope.