I would not wait--do the FWD. Use ro/di and adjust for temp (no need to adjust ph). A powerhead and a small heater in a bucket with ro/di is all you need. Place him in the bucket and use the powerhead to push water around the mantle edge and then leave him in the water for a total of 15-20 min. I am in no way a clam expert or a PM expert, but I have delt with the ccondition and am having some sucess. Make sure your tank water quality is in perfect condition, you have plenty of light and no excess iron in the tank (phosban (and other phosphate absorbers) has been said to leech iron into the water. Add UV if possible (pm is caused by a protozoan)
Good Luck
I have saved one (knock on wood) and lost one to PM
You may have to FWD more than once