Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your wee one’s situation, and wanted to help.
Again I do apologize that my colleagues could not aid you sooner, but can you tell me:
Any chance of head trauma during his seizures?
Does the lump feel attached to the skull or the skin?
Can you confirm he hasn't had a bump there or a soft spot since he was a pup?
Can you take a photo of this? If you can do so and post them online, it will let me see what you are seeing. To post them, you can either use the paper clip on the tool bar. Or if you cannot see that on your phone/computer, then you can post them on any site (ie Flickr, Photobucket, Imgr etc) and paste the web address here for me to have a peek.