You can't or he wouldn't be straight. That is true, but what you can do is make him want something you offer more than his aversion to doing something that might be considered gay.
Assuming you are speaking about a specific straight guy...you can not make a straight man gay. However everyone is somewhere along the line from extremely straight to extremely gay....if (you believe that) the guy is close to the middle you might ask him how he feels about gay men in a non-threatening way. You never know what his answer might be. However, if you are certain that he is indeed completely straight I would advise that you do not take the chance of ruining your friendship by making a move that would certainly be rejected and possibly ruin things forever between the two of you.