I'm guessing you would be well advised to start the process anyway as if you had received the watch. In MyEbay purchases there will be an option to contact seller against the item.
That will bring up a list and one of them is the item not being as described. I think if you tick that it will register the item as being disputed on Ebay and the seller will be notified and given a timeframe and options to respond.
I would do that and send him a message separately re-stating that there is no point him sending it as you have opened a case for mis-description which Ebay will find in your favour on given the clear mis-description and he will be left out of pocket with postal charges if he still chooses to send the item. He would be well minded to accept the mis-description and the "virtual" return of the watch.