Oh Charlie, you werer almost all better! What happened, there, buddy?

It sounds like the sore keeps healing from the outside in and something (thorn, germs, whatever) gets trapped and starts the cycle all over again. So it needs to heal from the inside out, like a puncture wound. In that case, you need to keep the outside soft and open while the healing takes place deep inside first. Soaking in epsom salts regularly would help with that.

Is there some little treat he likes, which you could put on a finger and keep him busy licking it off while you soak the paw? Maybe a tuna fish paste? Just trying to generate ideas.