Hello I just saw your post and couldnt resist to give you my toughts for I have had the same experience. I like this girl from high school until today moral of story i did tell her she rejected me we were and still best friends even after all of it happened.
Not every men is alike so are women and from my point of view as a man since he alredy has a girlfriend honestly would not be the most apropriate time to tell him but life itself isnt fear or is it? risk a long term friendhip is harsh but if he realy values that friendship like I did he will not break it. Consider the following:
What if he likes you more than u do like him, bout hes afraid to tell you just like u are it could be enouth reason for him to try finding someone else, beside I bet you would rather have him as a friend than not having him at all right? what if he thinks the same towards you? there is only 1 way to find out teling him. If he does respond he will have to make a choice betwen you or his curent girlfriend, as far as friendship I cnt say how it will end nor i can becertaing remember everyone is diferenet even when u think you are alike.. thats the risk you should chose wheter or not to take, if rejects you and decides not being your friend keep your head up and walk foward knowing he wasnt a real friend from the start.
thats my advise but the choise is your sweety, hope I was of help and for you to succed in whatever chioce u make have a great day...