What Does A Painful Lump On Head Indicate?

My 9 year old son has had a headache in the back of his head for 3 days, he doesn't have a fever but, today he developed a bump about 1/3 of a golf ball in the location it's been hurting. He hasn't been sleeping well. He complains he doesn't feel well and broke down crying at school today and needed to come home. I thought he slept on it funny but, now the lump concerns me. He hasn't had any accidents or rough play. A school staff member said a virus was circulating that caused pain in the back of the head at the top of the neck like his. His lump and pain are on his right side of the back. Should I take him to the walk in clinic?