I would agree with this. He could have sold the car to someone else in the meantime, but the buyer dragged his feed for 5 months now and wants another 45 days. Sorry, but that would cover the money you may have lost in depreciation and for storage of the car.
Tell him you were holding it for him on good faith and you had other potential buys which now you lost. Now you will have to relist it on ebay, etc…
Plus being that neither of you have paperwork, nothing to really go on. You have the upper hand because you got the money already.