Swollen lymph nodes in a 4 yr old?

I took my son to the dentist in Feb. he is 4 and he needed some cavities filled and needed a crown on a tooth, they gave him some laughing gas to ease his nerves. Right after we got home from the dentist I noticed a lump on the base of his neck, I figure maybe he had his neck wrong or because he was tense he has a tense muscle and I really didnt give it a second thought. Well not even a week later it was still there, well I noticed he had another swollen spot under his left ear on the same side as the other lump. I took him to the minor care center and all of his blood work came back normal and they said from him coming from the dentist and having that lump that he more then likely had an infection. They also said they may not ever go down, and after he sees his pcp they may want to biopsy them to make sure that they are not cancerous even though he believes they arent since they didnt just come up they were there right after his dentist appt. He also said that even with the antibiotics that they might never go down. Well he took some really strong antibiotics for two weeks and they went down a whole lot. But they are still there, due to insurance issues I have not been able to take him back to the doctor but I will be able to do it now Monday. I'm worried that they could be cancerous? He eats normal, he isnt sleepy, no night sweats and like I said all of his blood work came back normal even his plat. count. Could it be maybe that he still has an infection or could he just always have swollen lymph nodes. The doctor said that he they just came about for no reason then he would have admitted him into the er right then and there but because they came right after the dentist that they are an infection. Has anyone else had this happen or know anything about lymph nodes? I have looked on web md and have not really found anything. Any help would be great thanks