Good to hear that you will be getting him to the vet......We are in the midst of dealing with MCT...The way you describe the lump getting bigger, then smaller, just appearing,,worries me for your boy. MCT is a very agressive cancer.
I truely do not want to give you worries,,,I just want to stress the importance of ruling this type of monster out.
If and when it is removed (the sooner the better),,,make sure you tell your vet to get clear margins.
We'll be looking for updates,,,,and sending good thoughts as we wait....Hope it's nothing.