I think it's a really nice gesture of you and don't understand why anyone would be offended. Maybe the other moms would even want to join in and take turns bringing the snacks. That's how it's usually done---not sure why your league doesn't suggest that.
Anyway, the only thing I would suggest before you do this is to PLEASE ask the other parents if their children have any food allergies. You wouldn't want to bring something that a child couldn't eat or that would put him in any danger from exposure.
I'm a mom of a child with nut allergies. We thought he'd outgrown all of his food allergies after pretty extensive testing with his allergist, but after eating a pistachio earlier this year, he had an anaphylactic reaction. I had to give him the epipen, call 911, and he ended up spending hours in the pediatric ER. Not fun, but thank goodness I was with him when it happened, still had epipens around, and knew what to do.
Having two teams running around on the field is a lot more intimidating than just practicing with his own team. Your son just needs some time. He'll begin to feel more comfortable as he gets used to being around all the other boys and gets into a routine with the games. It's just all new to him now.
Good luck and hope you have a great season. We just had our flag football season opener today, and you're right---it's so much fun!
Best wishes,
J. F.