My oldest son is 21, so I understand.
Does he like melons? My kids love watermelon and cantaloupe. I know it's hard to get creative, because food can be boring after awhile. Boring food can lead to giving in to all those tempting foods. I have been a vegan for 2 months now, and I basically eat the same thing every day. It is hard. My kids are also eating mostly vegan (except when my husband fixes dinner or sneaks them out to get junk food), but I do fix them lean meats and whole wheat bread. Watermelon is yummy, healthy, and filling. My oldest daughter is picky and 15. I had her make a list of every kind of food she likes and different dishes. I went through her list (very short list) and picked out the healthy items. I try to have stuff she likes that is healthy. I also try to trick her into tasting things "by accident." I made a smoothie the other day and put food coloring in it. It had bananas and strawberries. I "accidentally" dropped spinach leaves in it and "accidentally" forgot to tell her. lol This kinda tricks work on adult kids and even my husband.