What Causes Painful Lump On The Head?

my husband had an accident when he was 6 years old and he had a plate remplacement on the right side of his head, now he is 46, and like two weeks ago I noticed a bump on his head, like on top of his plate like a golf ball size and he is complaning about headhache. He already got xrays and now he is wating for a CT scan diagnostic.