So in other words, he's dangling a carrot that's about two months away?

BTW, if you're dealing with "contracts" (implying a consultant/independent contractor relationship), then he's not your boss, he's your client. That's a big difference.

The main thing here is that he's asking you to commit to something that he's not willing to commit to himself. To me, a measured, non-commital response would be in order. Keep it polite and let him know that you'd be happy to discuss a contract for the summer once he's able to commit to one.

No point in aggravating the client but this is a business deal (selling your time and expertise). Treat it as such.