I'm going to ask my male straight friend who has a gf if he wants to date me
Why would you ask him if he is already in a relationship?
I've been friends with him since secondary school
That is not a good thing to do. It is wrong to proposition someone that is in a relationship. You will make them both mad at you since he is committed.
I don't care
It is even worse if you are gay trying to convert a straight person. You know how hurtful that assumption can be that you are choosing to be your sexuality, please stop. Respect their relationship and stay out of it.