I'll come at this from a couple of different angles - first as someone who was in PT for awhile to address low back issues that turned out to be caused by hip/glute issues and second as the parent of a 13 yr old who is 6' ~180 lbs and was around 5'6" or so this time last year (don't have his exact measurements handy).
As someone who has done the exercises and still does them today as part of a maintenance plan, there is no reason he can't do these on his own in front of a mirror. If you do a search you'll find a million different plans to help with this but he'd probably be fine doing body weight squats and then doing work for the adductors and abductors using a set of flexible bands you wear around your ankles or knees - doing sideways walks or monster walks. Since he's not injured right now it's really just preventative work.
So as someone who has been through it (and still lives it) I think it COULD be done by him, with your help, on his own. But, having a 13 yr old who we call Sasquatch because when he runs it looks like a one of those videos from a big foot sighting show, I think it makes a lot of sense to set him up with the trainer to learn the proper technique for several lower body exercises that can be done with low weight or body weight. It will potentially help with this issue (if there really is one) and it will set him up for future strength training which I'm guessing he's bound to end up in (since you're already going as part of team training). Between summer baseball and football season last year we did just that with our 13yr old to help with pre-football conditioning and to give him a series of exercises he could do on his own (work-outs that he could repeat). He enjoyed it and it set him up with habits that he's continuing with now as he goes to the gym to "work-out" whenever he has an off day from basketball.