Wow, that must be a horrible situation to be in. I don't want you to feel you have to give up bowling, but I will make a suggestion. Be ready, then if this bully grabs your shoulders, "accidentally" push him really hard with your elbow or something. The next time he does it, push him off, then turn around and "accidentally" punch him. Be sure to make it look like it was just a reflex (meaning you have to do it fast before you have a chance to see who it is!). Be shocked and be like, "Oh my God, I am SOO sorry! I thought you were a mugger or something!" (Note: Don't punch him until the second time he does it. Chances are you won't need to; after a good hard shove he'll likely have learned his lesson and stop or go pick on someone else.)
Hopefully he'll stop doing this. Everyone will think it was an accident (and who wouldn't be scared when someone sneaks up on him from behind, right?) and he'll learn that there's a price to pay for his bullying ways.
Hope this helps, and good luck!
Firewire - 1
Bully - 0
