So just walk right up to him and ask him to go with you. It shuldn't be all that hard because, after all, the two of you have a relaxed relationship and you get along well. If he assumes you'll ask him it's probably because you and he do have a good understanding. There's nothing malicious in his assumptions. Today I asked this guy I had a crush on to take me to his prom.
Tickets have already or are about to go off sale.
We are not even friends. My policy is never wait. If you wait, you may lose the opportunity. If you've asked him once, then you can ask him again. Once he knows you have a crush on him, his own feelings might change and he'll warm toward you.