There may be reason for concern if you think the player might be overdressing in order to produce more sweat and thus lose water weight. I've seen individuals do that thinking they will drop pounds. See if you can determine if that might be the case. If so, you may want to approach the parents again, talk to the player, or address it via a school counselor. If not, then the player may learn simply because he may be underperforming and therefore not getting as much playing time!
In my opinion, you have said your peace to the family now I would just let him be. Maybe there are other reasons he wears so many pieces of clothing, maybe insecurity, maybe cold.... so many possibilities. All you can do is suggest and the rest will have to be his choice.
Jen