I am so sorry to hear your son is sick. My son also tested positive for lyme but we think we got his early. He was 12. Anyway, I cannot stress enough the importance of getting to a lyme literate doctor. There is great controversy in the medical community regarding the diagnosis and treatment of lyme. I had a very hard time finding a doctor who would treat my son long enough despite the fact that both my husband and I fight chronic lyme.
I did however find a pediatric neurologist which was lyme literate who agreed to treat my son for 4 months with doxy until he was symptom free and he has been fine now for 2 years.
It sounds like your son has had the lyme for a while so he will need lengthy treatment.
The symptoms he is experience including the rib cage pain are very common with lyme but do not just brush them off. Take every precaution as you can never be too careful. Shrug off any condescending comments from ER personnel. Its your son, not theirs.
You may want to post a request for a lyme doctor in your area in this form and include "looking for LLMD in __________" and see if you get any response.
This forum was and continues to be a lifesaver for me so search and search and search. Being well informed about lyme is so so important.