Yikes! You are going to need some help to be sure he gets enough nutrients to grow and thrive. I hope you are seeing a specialist, and with a diagnosis like that a second opinion would be a good idea. You should also request a referral to a nutritionist if they have not already offered that.
There are some good books that can help you. One is called 'Healing the New Childhood Epidemics' it provides in-depth information on allergies so you can really help your child and be an advocate for him. Also look for recipe books on the autism diet, these are generally dairy and wheat free. Do a search on Amazon.com for these books and you can read the reviews too.
Keep trying the veggies even if he rejects them at first. It can take 10 tries before a child will accept a new food. Its important he re-learns to eat a variety of whole foods that are good for him. The further you can stay away from processed foods the better off he will be.
Also, peanut allergies dont mean all nuts- almond butter is an excellent food and full of good fats that kids this age need. Its rarely allergenic and it can be blended with rice milk for a healthy milkshake. Check with your allergist first though. Best of luck to you.