Swimming, gymnastics, track.
Some may call Gideon Beck an overachiever. Really, he just enjoys these activities.
During a gymnastics competition, though, a trick goes wrong and he hits his head. When he wakes again in the hospital, he has the mentality of a three year old, leaving his father, Quentin Beck, to care for him.
(Far From Home character involved. If you haven't watched the movie yet, I wouldn't suggest reading. It will contain spoilers. Involves CGL themes (sort of) and is completely nonsexual. It is not incest, bdsm, or boyxman.)