Should I put my arthritic rottie down?

I am sorry to hear of your predicament. At this point your dog sounds happy-I don't think now is the time. If he is unhappy, miserable,in constant pain etc-then is the time to consider that course. He sounds content at this point to be around you and may require leash walks where you can control his speed. This is a hard one to answer as you know your dog and we don't! But if he is still himself-let him be. You'll know when the time is right & it may be sooner than later. Good luck and continue to enjoy the time you do have with him. When the time does come remember that you have given him a great life and every opportunity (surgery) to help him.