Tell him that if the refund is not processed you will be filing a complaint. Then file one with eBay and make sure that you state that you have not received a refund as promised and the seller is holding it until you leave positive feedback.
I'm not an eBay policy expert anymore, but I'm pretty sure that the seller will get busted for this.
Since you haven't left the feedback you hold the power now. Don't, under any circumstances, leave that feedback until you are satisfied. If you do, you are screwed because you won't have recourse.
TBW
PS Does the seller have high feedback? If they are new, the negative will hurt them more...keep that in mind.