The first thing is to get proper advice on exactly what the copyright issue is, what was the seller paying you for etc.

If copyright did not pass to the seller and the arrangement was for single use, then you can inform the seller that you will make a charge (it has to be reasonable) for each further use and such use will indicate the seller's acceptance of the contract.

Should they carry on using the pictures, you send them the bill, if they don't pay you take them to court, as you can for the money they already owe.

I'm no expert, but without written agreement, can you conclusively prove that he did not take said photos himself.   If not then in my opinion you are stumped.

A court decision would rest on the balance of probability, absolute proof is not necessarily required.