I would talk to him, or the front desk at the gym to find out exactly how much he charges - that seems like a lot, unless you were pre-paying for a set number of sessions. You can also ask around at the gym to see if there is anyone else who would want to split the cost with you. I've been thinking about doing that with a friend of mine - she's had really good luck with her trainer, and her trainer said that if we both wanted to come to the sessions the cost would be the same and we could just split it.
A trainer can be an excellent motivator, and it sounds like he's a good one because he actually corrects your form. And if he's hot, well...