Does your dog try to run with the ball when you try to grab it, or does he try to play tug?

If he is running you should practice Come; Sit; Stay, then pet on the head.

If he is playing tug you should get a tug toy, play tug, and practice Out, or Drop, manually opening his mouth with your hand on his snout in the begining. Then Sit; Bite.

Try to seperate the game of fetch with the innapropriate behavior. Then when you play fetch use the commands you tought in a different situation and only accept the correct behavior. You can be more lax at first, and give praise, but as he learns what you want become more strict before you reward him.

Hope this helps....let me know how this works.