i had one die from cancer after it 1st took her spleen. she was in much pain and took her in for tests and pics which showed cancerous tumors throughout her abdoman. the vet removed her spleen and sent her home for another 6 months of living life and enjoying everything around her. we could have put her down then but she acted like it wasn't quite time yet...we had a great last 6 months together before the inevitable had to be done. what i'm getting at is, is he telling you he's ready now or is he still enjoying things? then the question is...can you afford to keep him comfortable during his end time?
it's always hard and you have my sincere sympathy...but if he's still enjoying his world, enjoy him while ya can...