I think you shoudl get a second opinion and do more research rather than just take one vet's word for it that you really don't know....not understanding why you didn't take him to your usual vet or at least get a second opinion regarding this before just agreeing to have him put down. I'd definitely get a second opinion before ending a pet's life, especially when they somehow diagnosed him with a painful cancerous disease just by opening him up to do a neuter. A vet would call about that to discuss options, never heard of one ho has the secretary call.