neutered male acting sexual

Question
I have a two year male cat, nueutered young. I have two older male cats about 4 years old also neuteured.  Well about a couple months ago I started noticing that the younger male cat is really annoying the older cats. It looks to me like he is trying to mate with them, he gets on their backs and bites down on their necks, needless to say this causes issues and then the older cat will fight back. Not really "fighting" but definitely will get away from the younger aggressor cat.  Is this normal? Is he really trying to mate? or is he just playing? He has not been spraying or anything since being neutered and uses the litter box normally.  

Answer
Sally,

Assuming he was neutered correctly, you can have the vet run a hormone titer on your younger male to see if his testosterone is correct for a neutered cat.  There are examples of cats having only one or no descended testicles, where the vet could not find everything to truly render the cat a neuter.  These cats will act as if they are not necessarily neutered.

If his hormone titer is normal for being a neuter, then he is playing.

Best regards... Norm.