When to put my dog down?):?

My labador dog is now 13. He was just diognosed with cancer in his front right paw. He still walks/runs and wags his tail, eats, drinks, poops/pees. But i can see a MAJOR diference in his attitude. It gets worse by the day. His eyes are bloodshot also. I love him so much and he is my best friend i sont want him to be in pain and i dont know when to put him down because he looks at me like he wants to keep living but i know he is in pain even with the pain meds we give him. I love him more than anything and the last thing i want is for him to be in mesery. what do i do?):