My Son Has Two Lumps On The Back Of His

My son has two lumps on the back of his head that have been there almost a year. I took him to an ENT, Pediatritian, and a Pediatric Cancer doctor about 9 months ago and they all said he appears healthy based on blood tests so they fre it's nothing of concern. They think they are lymph nodes that never went down. But, I'm not satisfied with that answer especially since they are still there. He doesn't have any symptoms, but neither did my husband when he had thyroid cancer. He only had a painless enlarged lymph node in his neck. Should I insist on a biopsy??