I live with an autistic teenager. Our household has gradually transitioned to paleo over the past 5 years (starting with her being put on a GFCF diet). She started out on the typical autistic diet, limited to a few carb/dairy heavy foods, and we were concerned at the start about her starving herself if not given what she was willing to eat.
She now quite happily eats ANYTHING. Salads, curries, soups, chili, slabs of meat, eggs in different forms, raw and cooked fruits and vegetables of many types... She likes these things. She requests them. She has not rejected a meal I have made in the past several years. Even disregarding sensitivity issues, it's been an undeniably positive step nutritionally. She's so much physically healthier now than she was when she started!
(It has not, however, been a magical autism cure. We definitely notice behavioral improvement. She's much happier, more cooperative, and aware of other people when eating properly - we can tell when she's been sneaking stuff by her behavior. But she's still autistic, regardless.)
I don't really have any earth-shattering advice. Start with the assumption that he won't starve himself, especially if he's already willing to eat some food that is acceptable. If there are paleo foods he likes, have them available to the greatest extent possible. You may have to make a lot of chili for a while :) Introduce new things gradually, and gradually stop catering to his food preferences (ie. stop having chili available as an option if the dinner you've made is something else). As the addictions are broken, he'll probably be more willing to accept new foods, especially if he learns that he has to eat what he's given.
I won't say that no autistic (or non-autistic) kid will starve themselves rather than eat non-preferred food, but that's a bridge to cross if you come to it, not something to stop the journey before it starts. If you are really worried, focus on GFCF first, and include GFCF processed food, non-gluten grains, and so forth while you work on expanding his diet.
School is definitely the biggest challenge, with all the well-meaning people who give her graham crackers "because they have graham, not wheat", or who don't supervise her well, then call CPS when they see her sneaking a sandwich or yogurt out of a trash can because they don't realize she's autistic and/or assume she must really be hungry because it's not typical "treat" food, or when the art teacher decides they need to do still lives of boston cream pies or that the (high school age) kids can't survive a 20 minute bus ride without a daily snack. (Yes, those are all real-life examples that have happened to us!)
But you can only do what you can. None of that is really caused by the dietary restrictions - it's just that the dietary restrictions make us really aware of how much school can undermine parental nutritional choices!