Hmm, I've got no idea. Does it seem to bother your kittten? Any scratching at it or pawing the ear? If you touch it does it bother him/her? Just gathering info in case anyone else has any ideas.
Hi. That's on the back of his ear, correct? Is it hard or soft? Can you move it around with your fingers or does it seem to be immobile? It could be any number of growths - cyst, wart, tumor, etc. Perhaps, your vet would be willing to take a look at a pic you send to them? They might know what it is, or they may tell you to let them take a closer look at it. If the vet tells you to just keep an eye out on it, then take pics of it so that you can monitor any growth or other changes.
Since he is seeing the vet next week, having them check it out then is probably good enough. But, still you might go ahead and take pics of it between now and then, in case it does grow/change - then, you can show the vet the changes (if any) you see in it.
I may be seeing things or it could be my computer screen but it looks like some type of bug is imbedded in the skin. Regardless, I'm glad you have a vet appointment next week and can get it checked out. Good luck, please keep us posted.