Please don't take this wrong but I hope he can't play till 10 or so; Lacrosse and Rugby are just too physical for someone that young. Consider thats the only body he is going to get and the fact that he is aggressive enough @ 5 to appreciate a great sport like Lacrosse, probably means he is going to be able to compete competively in T-ball basketball and soccer. These sports are available in his age division and are a lot less likely to put him on the sideline as a wounded warrior when he is old enough to play the sport he loves he will be conditioned and ready to be the player he dreams to be. I have coached for 15 years and can tell you I have seen too many start too hard too soon and not get to there goals because of injury. And of all things we want our kids to grow up with unfullfilled dreams and resentment of the adults put in charge of them are about the most destructive to there development.

Start him right now, I would go with like having a family member just teach him how to play untill about 9 or 10 and put him on a team and he will be amazing if he sticks with it.