Something like 1 in 4 diabetics also have celiac markers (which means they should probably be avoiding wheat). I just read that somewhere but I can't think of where I saw it or I would attach a link. Perhaps google it.
My dad has been a type-1 since he was 18. One of my nephews only made it to 6 before he developed it; another nephew (different sib) has made it to 10.
They don't really restrict their diets but the 2nd nephew was told by his endocrinologist to watch gluten. They also try to avoid simple, highly processed sugars.
I have a kid who is severely gluten intolerant (but not diabetic).
With the autism it will be even harder to get control of his food, but it's probably even more important to do so now, just from a purely nutritional standpoint. Have they met with a nutritionist? That might help put a little "fear of God" in them. A good one will explain how vitally important it is.
Good luck.