Do not pull on him. You can damage the bysal gland by pulling on him. Is he attached where you can access the bysal area? If you can access the bottom of the clam, you can CAREFULLY tip him to the side and cut the bysal threads with a razor blade as far away from the shell as possible. There is no other way to safey remove him from his rock. If you damage the bysal gland he will not survive. After you romove hime from the rock, the best thing to do is let him attach to an empty half clam shell and then place the shell where you want him to be.
Good luck with moving him.
Mary