Rather than insert yourself in the middle of his current relationship, let him figure out how to end it amicably with his current girlfriend. You don't want to be falsely blamed for their breakup and bear the brunt of all the bad feelings that come out of that. Skip the drama, especially if there are games involved. It's HIS responsibility to handle this interpersonal matter himself without your assistance. All he needs to do is have one difficult face-to-face conversation with his current girlfriend and it's over.
Tell your guy friend that when he is finally single and available to date THEN comes to you to discuss the future of your relationship. Take it slow, however. His seeking to jump right from one girl to another should raise a caution flag.