old dog with new dog

Question
Hi, I have a 13 yr old Maltese who is having hip and back problems. We have put him on Rimidyl and a stomach medicine and he seems to be getting around easier and actually playing with our yorkie puppy (who we got in June and he had nothing to do with him until now. The puppy is currently 7 mo old). The problem is, he is 'playing' too much. Chandler (the Maltese) has been fixed since he was 3 yrs old but all of a sudden he is like, humping on the Max's (the puppy) back and actually finishing, like, wetting it. I know sometimes when people are getting old, have a head injury or in the hospital for a long period of time they sometimes become oversexed, do you think that could be his problem? That he just likes it so he uses the puppy? Maybe his meds are causing it? Or maybe it's because Max is at the age where their testes drop? I'm not sure at this point if its a territory thing because he did that at the beginning but didn't 'finish' and was a bit aggressive towards him. Now he is nice and wants to play with the puppy, not aggressive at all, even sharing his food. He does it constantly, I am having to give Max a bath 2x a day and try to keep them separated. Is there anything I can do to stop this? I thought it was a phase and wouldn't last but it has been at least 3 weeks now. Thank you for your time.

Answer
Humping behavior is normally rank related (dominance) and doesn't usually involve a sexual act.  I'm unsure what you're seeing, but it certainly sounds unusual.  There may be some neurological problem involved, or a case of diminishing cognition (as in humans, dogs develop dementia also).  Dogs can be sexually active after neutering.  It's possible that what began as a rank statement has become something else altogether.  Do not allow this interaction.  Distract both dogs and separate them for a few minutes when you see it.  Meanwhile, bring this problem to your veterinarian.