Wow, he's really dealing with a lot. IBD as well as Celiac. As far as the Celiac goes, some people do get pain as a symptom. How quickly it begins to subside after going gluten-free is pretty individual. I had stomach pain that felt like I got kicked in the stomach, along with nausea, and lower cramping due to the D.
The D began to ease within days of going on the diet, but the rest of my symptoms took longer--I would say it was 4 months before I can say I "turned a corner". I also had additional food intolerances and neuro symptoms that delayed my recovery even more.
All in all, it took me about 18 months to feel better. For some, it's more immediate, and for others, it takes even longer. That depends on a lot of things like how long the damage to the intestine has been happening.
With the diet, he wants to make sure to check any vitamins, medications, and personal care products for gluten. This includes chapstick and his wife's lip products.
In the kitchen, his toaster, collander, and any wooden boards and utensils need to be kept separate. Non stick cookware is fine as long as there are no gluten-harboring scratches. Glass, ceramic and stainless steel are fine, also.
If the rest of the family eats gluten at home, his foods must be kept separate--one crumb will induce a reaction. The same goes for "dipables" like margarine tubs and jelly. It's safest to keep those separate, too, to avoid contamination.
In the beginning, it's a good idea to be dairy free as well. The damaged villi can't properly digest it. Maybe eat just whole, natural foods, as opposed to processed.
I don't know how long he's been gluten-free, but sometimes people who are intolerant to gluten can be intolerant to other foods as well. Soy, corn and dairy can all cause problems.
I can imagine how hard it is for you to see your son going through this (I have 2 sons, 26 and 22). I'm glad I'm the one going through this and not one of them. Anything we can do for you, just ask.