Interesting note. I suppose he may be right, but I'm not sure I would be so quick to conclude it is a worthless coin. A couple of notes in my rather short but very compressed time studying and collecting English coins.
I'm somewhat surprised that "doesn't sound like silver" is your dealer's reason a counterfeit. I would think a more learned numismatist would measure and weigh a coin to have a better estimate. I would also think that the planchet is so thin on the shilling, so it may not have the ring that one would expect from the larger and more formed milled coins of the later years.
If it is indeed a counterfeit, then the next question is to ask when was it counterfeited. Was it one of those nefarious forgeries from China, a contemporary replica, or a counterfeit from the same century the coin was minted. If it was counterfeit from the same period of time that the coin was minted, then it may even be a more valuable find.
Just a couple of thoughts. I look forward to any other points of view on this.
Thanks!