I think it depends on the nature of your relationship. Personally I see paying for the Prom the same way I do dating. If I ask a guy out on a date and he accepts, I pay. If he asks me out he pays. After that we can go Dutch or switch off who pays.
But if you know the guy doesn't have a ton of cash, it would be great to split the cost, even if he asked...
It depends on the situation. Most of the time, the guy pays. But if you're not too serious together, and you're just going as friends or as a date, then usually the person asking pays. Or you split it.
Best way to figure this situation out is to go up to him one day and hand him some money saying that it is for the prom ticket. He'll either take it and go get your tickets, or he'll refuse and say he is paying. Takes all the guess work out of the situation, and makes you look nice if he was going to pay anyway.
Im paying for my own ticket this year because prom is still a month away and I never know what might happen. If you haven't been together long that is probably your best bet if you really wanna go. It also wouldn't hurt to ask him what his plans are.