i did gymnastics for years and years and found it really useful although i have a few injuries from those days!
it made me dead strong so when i went through puberty and my eds flared up it wasnt that noticable as my muscles were strong enough to deal with the extra bendyness. i'd pay extra attention to any injuries that he may develop and rest until they are healed- i didnt rest and trained through my injuries and so i permanently damaged some joints. oops!
i still do a lot of the conditioning that we did at gymnastics and it's one of the main reasons why i'm able to walk as my SI joints and my hips are so loose i'd not be able to stand up if i wasn't so strong. so i think its a great idea that he's starting gym as he'll learn exercises that'll keep his joints nice and stable. maybe it'll be a good idea to write these down so he can refer to them later in life if he chooses to stop gym? i've forgotten some of the ones i used to do and i'm rather annoyed at myself for not making notes when i had the chance!
it also helped my balance, posture and coordination and i dont fall over as often if i remember to do the little balance & coordination exercises from my gym days (dead basic stuff like closing your eyes, lifting up a foot and trying to touch it with your hand, it's amazing how much that one helps.... dunno why though!).
it aslo taught me lots about the importance of stretching and gentle daily conditioning as we all learned our new skills faster if we did our conditioning and stretching at home inbetween our training days, and we also advanced on to new, cooler skills faster so we learned to do them everyday and i still do now! it's become a habit that i intend to keep for as long as possible.
i hope he has as much fun as i did at gym!