My son has lumps on the back of his head and neck and a few behind his ears but he doesn't have insurance so I don't want to take him to the hospital unless absolutely necessary

Could be something-- or nothing serious. But it can't be diagnosed here.
Any urgent care or clinics in your area that would be less costly than ER? By whatever means, he needs to be seen by a physician.
You may even get some advice if you call the ER or one of the clinics.