Next time he asks you, just say, "No Thank You".
Leave it at that. do not engage him further. Don't bring up that you've discussed this before. Just walk away with a no thank you.
Since you've already discussed this issue and he's refusing to accept your sobriety, don't engage him any further. Keep refusing and simply reply, No thank you and nothing further. Don't argue with him.
No thank you. I do not drink. Should he hand you a glass, pour it down the sink and say No thank you. I do not drink.
No point having the same argument over and over.