Also, if $400 was enough to break this kid financially, how was he possibly going to afford membership in the long run? I'm thinking he was having second thoughts about joining, and used this as an excuse.
I agree to talk to him casually, but keep in mind that he may just be making an excuse to get out of the pledge program. We had a kid who got in a car accident and ended up owing a substantial amount and that didn't even cause him to break a sweat regarding dues. He just found a way to raise the money himself (and this was a kid who pays for his own dues). While I'm not saying the two situations are identical, I'd be careful what you say so as not to look desparate, but make sure he understands you still want him in the house. For a kid who may be on the fence it's important to keep a balance between the two.