Now that you're convinced that it's okay for you to ask him, that it's also okay for you to pay for him, and now that you know what you want from your date, we can move on to ways and means for you to ask him-- that is, catch him and make him your date for prom.
First, let me say that as far as the ways of asking him go, there are thousands of ways, and the only limit is your imagination. But because you might also need a few suggestions, I've provided the link below so that you can read about the hundreds of ways in which others have asked someone to prom. You obviously don't have to do exactly what they did. You can modify what they did so that it best suits your character. You can even select the type of approach they used so that it suits the specific relationship you and your guy have.
Below, we will focus on some of the common problems girls have in asking.
Solution: If you're uncertain whether he'll say yes, it's usually because you like the guy and have romantic intentions, but you are worried that he doesn't like you back, which is why he "might" say no. There is only one solution for this problem, and that is: ask him at once, right away, pronto. Why? Because the very worst thing in the world is not knowing. If you ask him, then whether he says yes or no, at least you'll know exactly where you stand. And if he says yes, then great, you just go right ahead and start getting ready for prom. And if he says no, then you can at least move on, and go and ask someone else, so that you at least won't be dateless come prom night.
Solution: If you're really that shy that you don't dare ask, but you know you must ask anyway, then here's your option: you have to put yourself in a situation where, once you've started the process of asking him, you won't be able to back out. This way, come hell or high water, you will have to ask, and you will have to wait for his reply--and no, the earth will not swallow you up, no matter what. So how do you put yourself in such a situation? Quite simple: avoid asking him face to face. Instead, do something-- anything--where what happens is that he finds your request, whether on paper, or written on a blackboard, or hidden in a balloon or arriving in the mail, and then he has to reply to it. The point here is that your shyness is preventing you from walking right up to him and asking. So if you avoid the walking-right-up-to-him bit, you can get around your shyness without actually having to overcome it.
Fear of rejection.
Solution: Think of this exactly as you would think about the uncertainty problem above. You don't know if he likes you, and there's a good chance he will say no when you ask him. So? The important thing is that once you ask him, no matter what he says, you will know exactly where you stand with him. This will allow you to move on if he says no, or to start making prom plans if he says yes. Two important things about this: one, at least you will know where you stand with him, and two, you can be proud of yourself for having conquered your fear and asked him. Besides, so what if he does reject you? It might sound silly to say it, but the truth is that there are alot of other guy-fish in the sea, and that the eschaton will not be immanentized (end of the world won't happen) on account of rejection.
Afraid of looking dumb.
Solution: Wait, do you honestly think that you're going to look dumb and that he's going to laugh, or that all your friends are going to laugh, or that the teachers and even the school-crossing guard and the bus drivers are going to laugh if you ask him and he says no? First of all, he won't laugh and you won't look dumb because even if he says no, he will be flattered that you asked him--who wouldn't be? As for everyone else, they probably won't even know that you asked him, and probably won't care if they did know. What does it matter to the bus drivers or the school-crossing guard if you ask him or not or if he says no or not? See how silly it is to be afraid of looking dumb? So you go right ahead and ask him! Believe me, you won't look dumb. The only way you will look dumb, is if you don't ask him because you're afraid of looking dumb, and therefore lose the opportunity to be with your hot guy on prom night. Now wouldn't that be dumb?
My friends will think I'm nuts.
Solution: Who cares what your friends think? Let's say, for example, that you want to ask this one guy to prom, but your friends either don't think much of him, or think you're crazy for wanting to ask that particular guy? What I say is, why should you care what they think? Aren't all your friends going to prom with the dates of their chocie? So why should what they think deny you the same privilege? In other words, if you listen to what your friends think, and not to what you want, then the only one who loses out in the end is you. Think about it.