Prom?

So my boyfriends a senior and im a sophomore. He goes to a different school but i know his proms in April. He hasn't asked me to go but he has mentioned in but not in a im definitely going with him way. He has joked around saying he was going to take someone else ( just to piss me off) but i dont want to come right out and say it but if im going i need to dress shop what do i do?