My son, 7 months old, has a smal pea-sized lump on the back left side of his head. It is not fixed in place, I can move it around with my finger. I do think it's attached to the underside of the skin. What is it? Should I be worried?

My son, 7 months old, has a smal pea-sized lump on the back left side of his head. It is not fixed in place, I can move it around with my finger. I do think it's attached to the underside of the skin. What is it? Should I be worried?
I would take him to the doctor first thing Monday. It may just be a cyst. I wouldn't worry too much unless it is bothering him or gets bigger then I would take him to Urgent care or ER.