If he has it in his mouth for extended periods of time, it will start to affect his speech and palate development. This happened to us with our younger daughter at that age, and our dentist told us we had to get rid of the binky immediately. DD wasn't ready to give it up, either. Our wonderful nanny gave us the best advice about it - simply cut the tip off of all the pacifiers. Keep giving them to the child as requested, but they'll very quickly give it up on their own. Well, I thought that was crazy advice, but tried it. My daughter stood there at the binky drawer (yes, we had an entire drawer of them) and tried the binkies one by one. Finally after the last one, she looked at me and said, "They all BROKEN!" and wandered off. She never asked for one again. It was the strangest thing - but our nanny had said that was what was going to happen, and it did.
I would say if your son is chewing them, get rid of them. It's a safety issue at this point.