I'd suggest that you tell his coach you'll be glad to be an extra adult on the field, and that you'll support him any way you can. When the time is right, let it be known that coaches should an advocate for every player on the team... daddy ball has turned off too many good kids from sports.

Teach the basics. Make sure the kids know how to properly catch a ball. Their natural instinct will be to catch it with their glove facing palm up, which can let the ball jump into their face. They'll also need to learn how to run the bases. Keep it positive for all the kids.