I agree with ree4tu1 ....
And I would also have the doc explain why s/he is not testing for coinfections that about half the time are carried by the same ticks that bring Lyme. The coinfections can be just as bad as Lyme and need different treatment.
The symptoms of the coinfections are sometimes pretty distinct from Lyme and the doc can figure out what to test for to confirm the coinfection.
If the lyme is in the blood, the body is still trying to make antibodies to it, so the immune system is working, but I am not certain the immune system alone can eradicate the disease. In late stages of lyme it can go out of the blood stream and likes to live in the body's tissues like the joints. In my opinion, your son should be treated for lyme as a precaution. Treatment in early stages is very important and much easier. I am not sure why the doctor decided to not treat. Lyme symptoms do cycle at first about every 4 weeks, so be sure he isn't having flare-ups.
Lyme disease is a bacterial infection carried by ticks that may also carry other diseases. The tests and treatments are different for the various diseases.
I am not medically trained and it may be possible that all the Lyme bacteria can be killed by the immune system and we just dont know it has happened, but there are so many people sick with Lyme that I would get treated just to be sure.
Did your son's doctor advise against treatment?