Probably you're going to want to go wireless. A couple of things:
1. Get either the countryman B6 or B3 - I'm presuming that the player will probably sweat, and these are the most waterproof of all the lavs.
2. Get some non-lubricated condoms to put the Tx in. Again, for sweat protection.
3. Get something sturdy to hang the Tx on. If the player is running and moving a lot, probably his pants are not going to be able to handle the extra weight. Trew audio sells body pack "mounts" that velcro either around the thigh or the chest and are well thought of - I'm going to be picking up a 4 sets before my next shoot.
