Do we have to pay the mechanic if they can't fix the problem?

Our Honda had a major transmission problem and we sent it to this mechanic which, after 3 months of rebuilding and installing and such, still can't fix it. They figured that it was an electrical problem and he really doesn't specialize in that. The problem is, if we decide to tow it out of his shop and into the actual Honda dealership, do we still have to pay him? We haven't talked to him yet, but we really don't want to pay because he didn't fix it, despite the fact that he probably spent a lot of money buying parts and new transmissions (he replaced the transmission three times) and labor.