Thank you for sharing your concern.
As you have mentioned, both of you seem to be in love with each other. As far as forcing him to leave his girlfriend is concerned, anything forced upon anyone does not last too long. Instead, try to talk to him regarding his true feelings for his girlfriend. Sometimes guilt can stop us from breaking off a relationship even if it has ended from within us. However, under the circumstances it would not be a fulfilling relationship for long.
In the mean time, you need to have your priorities clear. What is it exactly that you are looking for. Is this situation alright with you, would you like to go along while he is also with his girlfriend, will it be a fulfilling relationship.... Introspection, and open and honest talk with your friend will help you find answers.