My Son Is 4yrs Old And Has A Left Lateral

My son is 4yrs old and has a left lateral neck lump, about the size of a nickel. Went to Dr 2 weeks ago. They weren't concerned bc it was mobile. It has grown some and is more noticeable now. My son is not sick and has been well. Dr said if I just couldn't sleep at niggt, they could order lab work. Would lab work alone rule out anything serious like cancer? Or should i ask for an ultrasound as well?? My son says ithe lump doesn't hurt.