Lead Poisioning

My son is 5 years old,he is speech delayed and has a mild learning disability. he was exposed to lead when he was 2 years old. He had 21 ug of lead in his blood, the doctor tested his blood three months later and there was no lead in him, but I was told by a friend who is a doctor that lead can still be in bones and brain. I have been giving him vitamin c, cod liver oil, and vitamin B, I am thinking about having him do chelation therapy. Is there any other treatment that I can do???????????