My first thought is Celiac disease. My son came down with it when he was 13, although there was one symptom he was born with, no one related it to GLUTEN ALLERGY. When he went Gluten free, that sore went away, & didn't come back unless he binged. Gluten is in wheat, rye, barley. It's in almost all packaged foods.
There is a website, Celiac.com. I just checked it this morning. here is the link for the symptoms page. It can cause both ADHD & phycological problems including depression & mood swings. It's worth looking in to. http://www.celiac.com/categories/Celiac-Disease-Research%...
I had my son tested by his pediatrician, who knew it immediately, because Ashton was throwing up & had diarrhea. These are the obvious symptoms. I on the other hand had 3/4 of the symptoms list, but no throwing up since college. I suffered for 25 years undiagnosed. What happens (short version)is that the gut doesn't work right, then malnutrition sets in, then things don't work right. Brains included. Many suspected my son of ADD, but it was more of a learning disorder. After he became gluten free, learning became easier for him.Rather than filling the page here with type, if you are at all intrigued by this, we can talk further, I do have a couple other ideas. You are fortunate that he is motivated by the love of sports, he has something to look forward to.
May God bless you, A.