For a while now my male dog (blue heeler) will ask to be mounted my my female dog (blue heeler lab mix). He will rub up against her and then push his butt into her face. He will do this several times until she mounts him. He's only a year and a few months and has not yet been nuterd. It's very odd and he will do it many times a night.
I have a female Labrador Retriever. The frequency of her unconventional sexual behavior decreased after getting her spayed.
At around 10-11 months old she had her first heat cycle and started behaving quite strangely around the house, most specifically to her bedding (round bed about 4 feet in diameter). She would flip the edge of the bed up under herself and start grinding/humping it for 5 seconds, then jumping away from it while growling and repeat the process 1-2 more times. She would do this multiple times per day. The
act of humping most commonly is the action of a male dog, so it was extra strange to witness this.
Once we had her spayed, we thought the behavior would go away completely, but it didn't. She does however do it less frequently (1-2 times a week, that we witness).
She has been spayed for a year now, and the behavior has lessened but not completely gone away. In your case, if you plan to neuter him, it may decrease the frequency like it did for us.