C.V.
Check the sports and outdoors section of Walmart. They have all kinds of hunting stuff and I'll bet you could find some Realtree camo ponchos or something that you could hang up. Also, here's a site with Mossy Oak decor. http://www.partyon7th.com/mossyoak.html
Games?
How about "archery" with the rubber-tipped arrows and deer/ turkey targets? Use a traditional recurve bow. Local sporting goods stores sell them in kids' sizes. Prizes for the best shot?
Stength competition? Get out dad's compound bow and see who can pull it back the furthest (with assistance from an adult, of course).
Pin the white-tail on the deer? (use a deer target).
Sounds like it's going to be a great time! :-) Happy birthday to your son!