There’s a recent discussion that’s similar here: https://www.additudemag.com/forums/topic/high-calorie-small-quantity-school-lunch-ideas-please/
If he likes Periasure, try making your own protein shakes and pack in the calories. I use to make my kids milkshakes for breakfast with: protein powder, whole milk, full fat ice cream, peanut butter, and banana. I believe you can add powdered milk for even more calories.
The trick is protein and calories. A variety isn’t worth forcing, you just have to find what he can tolerate that gives him these high needs.
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism