If you live in a big city you could look on line for local meet ups and groups that are into raw vegan or allergy free living.
Meetup dot com is a great site.
Most of the larger high schools and colleges will have a youth for animals or vegan club that could help support your son when on such an extreme diet or when dealing with seemingly depressing restrictions.
I attend a local raw vegan potluck that meets at least once a month.
U Chicago's Celiac center hosts dinners and info meetings...even if he tested negative for Celiac it is known that most everyone can benefit from a gluten-free, alkaline forming, nightshade free diet that can help ease internal and external inflammatory issues.
Ask your naturopath/MD or immunologist if they know of group meetings or maybe form one using their office as a sounding board.
The local vegan and PETA clubs are always a valuable resource for people that eat restrictive diets by choice and could introduce him to kids his age that eat healthy and restrictive by choice, maybe helping him to not think of it as a source of Depression or restriction.
There are really great allergy free recipe resources like Cybele Pascal's blog, Mimi Kirk, Natalia Rose's book the Raw Detox diet has excellent easy to make recipes (just nix the Tamari and/or Nama Shoyu), David Wolfe is all over the board with interesting (his interviews with others are sometimes a bit far out) youtube videos and excellent informative books - I <3 his fascinating, healing superfoods book.
Gnosis Chocolate in NYC is my go to splurge for healing treats that don't crash the healing process as is the Canadian Raw Foods Witch site hosted by a cute young raw vegan gal.
Another Canuk and endurance athlete Brendan Brazier has an entire line dedicated to gf/vegan health togo goods - the line is Vega.
Durian Rider is another very funny raw vegan athlete from Australia, his videos get hilarious but offer a wealth of info.
I hope it doesn't seem like I'm shoving raw vegan down your throat.
I've just found these diets to help me the most during my times of flare up crisis as raw is generally allergen free and if not, recipes are so easily modified to cut the Sugar and/or major allergens when looking for a healing diet.
Plus, finding local groups into such lifestyles may be a way out of his depression, by steering his focus into one of community and health from one of frustration related to obsessing over a major inflammatory response.
I genuinely know how he feels. Why do you think I comb and respond to these forums?!