One of my piggies had a tumour a good few years ago now. The vet suggested to me that I put him to sleep there and then to stop him suffering. He wasn't suffering in my eyes - he was bright, he was eating, he was running round with his ladypig like there was nothing wrong. The only sign that there may have been something up with him was the fact that he was sickly thin - he never gained weight after his diagnosis.
So we said we'll take him home, see how he does and the minute he starts to show signs that it's getting to him then we'd reassess with the vet and look at helping him on his way. He got another 6 months of wedded bliss with his ladypig before slipping away peacefully one night in his sleep, snuggled right up to her. I'm glad I never took that off him, or her.
Do what your gut is telling you. The minute he starts to look like he's in pain or suffering in any way then do the kindest thing we can ever do for our pets. But for now enjoy the time you've got with him and spoil him rotten.