Welcome to the forum jankrom!
You left out some key info here like what breeds the dogs are but with what is presented, here's my honest opinion.
For me this phrase is the most telling " I try to stop it but he just keeps doing it."
To me this suggests that the dog is not well trained and does not respect you as the leader. Especially if you are in the room, and you tell a dog that is getting aggressive to back off - and he does not listen? Dogs listen to the leader in my experience. Dogs are hardwired to please the leader in my experience.
I would suggest that the DOG thinks it is the leader and he is challenging your authority. This is the problem.
There is ZERO need that this dog be punished in a physical way. Proper punishment is done with the raising of a voice or other non physical punishments.
If this were my dog, i would be getting professional advice on how to become the leader. The dog would be enrolled in group obedience classes from a REFERRED trainer. Referred by someone you trust.
And it would be done asap. Aggression and potential aggression are dangerous.
here are 3 articles that may help.
Good luck!
http://www.pets.ca/dogs/articles/whos-in-charge/
http://www.pets.ca/dogs/articles/not...-is-free-nilf/
http://www.pets.ca/dogs/articles/dog...-dog-fighting/