What Causes Yellowish Sores On The Scalp?

my husband gets bad sores on his scalp. They look yellowish and infected. I have to pop them for him because they hurt him. In the early stages there is a huge hard lump under his scalp but in a few days the yellowish blister can be popped and it gives him instant relief from the pain. he has had this for years only along his back hair line. Now they have moved up to the top of his head and are coming down his forhead. What could this be?