older male dog mounting younger male dog?

[summer_breeze85]

My partner has a rottweiler/lab cross who is 11 years old, he can be quite aggressive to other dogs when out on a walk with absolutley no warning whats so ever. I have a 7 month old Border collie male and when we let them meet for the first time jake the older one wasnt even bothered in the slightest by jesse the pup. we have been supervising them spending time together so they can get used to each other over the last month, jake the older dog has been neutered a very long time ago but keeps trying to mount jesse who has not yet been neutered, could this be a dominance type behaviour? We will be moving into my partners over the weekend and have already come to the decision not to leave the dogs alone in the same room.