My question is this. If the horse can manage pain ok with bute, why is on 3 legs?
What I'm about to say, I am only saying because of the age of that animal in question. If he is in significant pain bute or not (which by the way, bute is not meant for long term use), if the horse can't get around without being in discomfort and time( a few days maybe a couple weeks) is not doing the trick then yes, I would really consider sending him over to the other world.
It's a different story had he been younger. I never would give up hope or giving him a chance but the main difference with this one is the age. The poor bugger has lived a long and happy life, let him go.