Sorry about your rat.
I just think you should let him hold out till he shows some signs of being in pain. I guess there is nothing you can do to fix it, I assume you've explored those avenues. I say, let him go on till he seems really sickly, then put him down (again, assuming you cannot fix what is wrong with him). If he seems happy, I say, let him be happy for as long as possible. Also, I would say, feed him extra treats, he only has a short time on this earth anyways.
Love him, coddle him, give him special snacks, take care of his vet needs as much as you are able.
Love him and appreciate him, and know that his time is finite, but that it was great while it was here.