First of all, what sort of objects does he put in his mouth?

Anything that is handy - baseball glove (while he's on the field), sweatshirt collar, etc. etc.

My son used to do this a lot. Apparently it's because he needs more sensory feedback than other people... so putting things in his mouth gives him this. It's called sensory intergration dysfunction.
My son still has this tendency but he understands it and doesn't chew the collars of his shirt anymore!!!! Also, the more sensory feedback he got in other ways, the less he needed to chew things!!!
In some circumstances (ie not when they are playing sport)chewing gum can help!!!!!
My son is only 11.... and it isn't a major issue anymore, it just drives you nuts at the time!!!!!

Thank you! We'll try the gum trick. I'm glad to know it's not terribly unusual and there's even a term for it. Any other ideas besides gum, mints, etc? This is definitely helpful.