Safest way to protect your kids is to:

a) don't leave them alone with other adults 1:1. Obviously the age of your son will determine just how carefully you would have to watch them, but if your church is like ours over here, youth leader aren't allowed to be alone with kids anyway. If you think the wrestling is going too far... I'd suggest having a word to the guy and saying you don't want him to wrestle with your son like that (make it friendly and let him know what your boundaries are). If he knows you are a concerned parent and are taking an interest he won't be likely to try anything inappropriate with your kid.
b) educate your kids about how to say "no"
c) talk openly with them about their lives so that if something's going on they will automatically tell you about it.
d) get to know the adult caregivers in their life so that you can talk openly with them too. Make friends with this guy... like Godspell says.

there's my 2 cents worth.

Blessings
Cat.