How do I reply to a professor whether I accept his postdoc. offer?
I think you are thinking of it backwards. You should use this offer to try to get an early decision from the other professor!
You can say that you're very interested in their position, but you also have another competing offer. You could ask them to make a decision, or give you a timeline for when a decision would be made. Then, to the professor you have the offer from, just write a polite e-mail thanking them for their offer. You can be honest and say you're waiting on a competing offer, or you can say something vague like you need a few days to consider, and give them a deadline (probably no more than 10 days) for your decision.