My adult male just started to trying to mate the male puppy?

I have had the Adult male since he was 8 weeks and he is now almost 6 years.
I also have a male puppy now 12 weeks of age, yesterday I took them both to a friends and the adult dog left first and I stayed at my friends for about 3 more hours and when I returned home the male is now trying to mate with the puppy. He hasnt done this at all prior to that and I have no idea why he started this.

Any advice would be appreciated.

Sounds like the adult male is trying to dominate the puppy. Just try to distract the adult when he goes to do it again.

Yes, he's just proving to the pup that he's still in charge despite the fact that they were apart for a while.

I'd stop it whenever it happens because it can lead to 'arguments' later on!

Agree with what has already been said

Okay thank you, we have been making him lay down by us everytime he tries it and hopefully he will get the point.

Much appreciated