This is one of those cases where I am simply not comfortable with the OP or the elderly gentleman making a decision based on the advice of an internet forum. Particularly one where the advice is so definite and so much money is involved.
I firmly believe that a pause needs to be placed on everything until someone has consulted an attorney on behalf of the elderly gentleman. Nobody even knows at this point if the check is any good. If the attorney believes that a contract has been made and the check proves to be valid then the deal may have to continue as is. If not, then the opportunity has been taken to preserve at least some of the assets.
If the dealer is on the up and up, the dealer won't mind the scrutiny.