@oscar6,
Oscar, it sounds as if this guy is not sure about himself if you believe that he is either bisexual or gay and on the down low.
There is a possibility if he is not entering into a relationship with you but wants one, that you are his cover at least for himself as he's coming to terms with what he wants out of life.
I would become his "best" friend and let him know that you are aware that he needs to find himself and you can't be a part of anything sexual or emotional with him anymore but will be there as a friend.
If he is in a state of confusion of his sexuality, there can be nothing between you two, nor should you want there to be, as he can't give you 100%.
I also agree with Jespah it is possible he is experimenting his sexuality so please get tested and be safe.
There are plenty fish in the sea, take control of your life, don't allow others to control it, it's your life.