What do I tell him?

I started going out with this boy. We agreed on almost everything--except The Winter Ball. I asked him if he wanted to go and he said he wasn't into that kind of stuff. So I lied like I never wanted to go. Then my friend from elementary was looking for a date. Everybody rejected him! (He's a little chubby) I felt bad for him. He's not ugly or anything. He's just fat. So I asked him to go with me. Now he's all psyched to go. I was too. But then I overheard my 'boyfriend' talking about how he worked a LOT to get some money to buy a tux. It turns out that he wanted to take me, but his family can't really afford stuff. So now I feel kinda bad. How do I let him know I'm going with someone else? Or do I dump my other date?