In my personal experience, i tried the nutritionists diet that you speak of, and i will fully say that it did nothing for me, didnt get worse, didnt get better. I also tried the SCD stuff and had no luck their either, i know some people have had luck on the diet, so i am not going to bash it since everyone is different, but for me, it was a waste of time. I went from 240lbs to about 195lbs at the peak of my disease. The two above diets i was unable to gain a pound back, but within 3 1/2 months of my own personally tailored diet, i was back up to 235lbs
Also on a very opinionated note, you can get lots of calories and gain weight WITHOUT complete trash like fast foods and over processed stuff you get from the grocery store junk aisle. I am very heavy into body building, especially in the last two years since i was diagnosed, my diet is between 5000-6000 calories a day, and that is ALL clean cut food, hardly any truly processed foods, other then breads and dairy, which isnt really processed that much at all. Ever since i have begun my own diet i put together, along with a training regime that includes 6 days a week at the gym, cardio included and lots of sleep, i feel awesome. I also play hockey two nights a week in a league based right by Toronto. This year i have had the best season i have ever had in my life, pre or post crohns (finished 3rd in the league in points out of 260 players), so its safe to say i dont think i have ever been healthier or more in shape in my life.
I guess my overall point is that there is never a set diet, medication or quick fix, you have to figure it out on your own (in the case of your son since he was unlucky enough to be cursed with this stupid disease at such a young age which is completely unfair) he will probably need a little help on figuring it out. But if one thing doesnt work dont be discouraged, it may take a couple rounds to find that perfect combination, but i believe with crohns we all have some kind of lifestyle that can control our disease well enough to live a healthy lifestyle, it has worked for me so far. I wish you and your son the best of luck on finding the diet that works best for him.
P.S. I know doctors are almighty and knowing, but dont listen to everything they say when it comes to diet. My GI said NOTHING that i do with my diet will help my crohns as it is not caused by diet. If i ask anyone here though they will tell you diet effects them big time, including myself. To this day still, no matter how great i feel, if i eat any of my "bad" foods, which the two big heavy hitters being Onions and Pork, my internal system goes INSANE and i end up having multiple BM the next day. So my doctor is obviously ill-informed
P.S.S. I consider Green Tea (imported, not the crap tetley makes) to be a godsend, ever since i started drinking it religiously 6 months ago (never drank tea in my life) i only have one BM a day with no D. I am on Imuran, but it the tea made a night and day difference in a matter of 4-5 days. I now drink 3-4 cups a day and find it works wonders.
I apologize for getting a little ranty above, CD pisses me off sometimes and i get a little worked up