I am one month out from the j-pouch take down and I agree with your son. There are days when the butt burn is so bad I am afraid to eat.
On those days, I stick with a liquid only diet--mainly Ensure Protein Plus Vanilla. There is also a lactose free version. The case you have may be lactose free. If it is, it will say so on each bottle. It looks milky but does not contain any dairy product.
Chocolate, sugars, fats, dairy products, red meat and gluten products go right through me with a very high acid content. The rest of the time I eat a low residue diet. You can find a good description at mayoclinic.com.
My breakfasts include gluten free waffles, gluten free potato pancakes with applesauce, oatmeal, soft boiled eggs, or gluten-free french toast.
Lunch is usually soup with a very simple half sandwich--ham and cheese, peanut butter and jelly, tuna fish, etc. No spices or mustard.
Dinner is tough. I do fine with roast, poached or broiled chicken and fish without sauces. White rice and a limited amount of potatoes is ok. Pasta Carbonara without ham is ok. Fried rice with chicken and vegetables is ok. Carrots, spaghetti squash, zucchini without the skin, and peas are ok. Gluten free pizza is not ok. I think the fat in the cheese is too much right now.
People tell me the food problems are temporary and that what I can eat will expand over time.
Desserts: Jello, sorbet, gluten-free cookies, sliced bananas with a dab of peanut butter, seedless watermelon. I am not a big dessert eater.
Other things that helped are a 1/2 dose of Questran twice a day and Immodium before eating every meal. If I have an acid or upset stomach, Pepto Bismal helps. I clean with Balneol on tp. It neutralizes acids and coats the skin with a mild protectant. This works better for me than our bidet seat or Pamper wipes which left me a little raw.
Finally, I put Calmoseptine on a small gauze pad and leave it on the effected area. That gives immediate relief from the pain. I take a vicodan before exercising because exercising is really important to getting the digestive system to work well. I walk and lift light weights. On a really bad day 2 cups of epsom salts in a bath is heavenly.
Rectal exercises helped my continence. Imagine clenching and releasing a pebble in the rectal canal. I do about 60 a day. Every now and then continence is still based on how fast I can run.
What works for me may not be what works for your son. It will take time and a lot of experimenting to find what works best for him.
My heart goes out to you and your son. Please let him know he is not alone and what he is going through is not uncommon. This may make him laugh. Another name for butt burn is Monkey Butt. There is even a Monkey Butt Powder sold in a lot of equestrian magazines and Ace Hardware stores. The monkey logo has a giant sore, red bottom.