You should know by now that straight guys love the feeling of being taken cared of and being appreciated, better start from there. He knows you're a good pal because you are always with him during boxing matches but extend that gestures by cooking for him his favorite meal of buying him that batman action figure for his toy collection. Try to impress him, make sure that you are sending a signal that he is special to you.
Although don't expect much from a relationship like this. There is still a tendency that he will go back to the arms of a real girl. However, that should not stop you for making a mark to his life. A relationship is not a gauge if he truly loves you or not. As long as you know that he has the same feelings fo you, that should be enough for now. You have to be patient for it to run smoothly. After all, he is still cornered by a society that is conservative and is not ready for an all out experience like what you want to have...