First of all, I see that letters of two years old are still usable, although I don't know what's the hard rule about it. Secondly, if he wants to sign now, let him sign it. Later when you are ready for submission, you could very possibly ask him just to resign it again, if he's been nice. Or you can just say that you may be done with your preparation next year, and ask whether he can sign then? Personally, I'd just let him sign now and use the letter later.