I don't know at what age it stops being play but T is a year old and today we met up with his mum and some sisters. He took a real fancy to one of them and was constantly trying to hump her. It definitely didn't look like play 
What should i do in these circumstances?
Today, we grabbed his collar and let the rest of the pups carry on playing. After a couple of minutes we let T go again but instantly he was on her yet again. I put him on his lead for a bit longer but again he went after her the second he was let off!
I'm worried that he will now continue to do this when he is off lead.
Neutering is not an option I'm afraid but I'm happy to put in the effort of training. I just need to know what to do. I know neutering would not guarantee to stop the humping anyway, but I just couldn't get him neutered.