looking for help

My 5 year old was diagnosed in September with Juvenile Rheumatoid arthritis. He was also a preemie who has 5 blood transfusions to survive.
His liver and kidney functions have been elevated. He has been tested for HIV 3 times and all negative. He has been tested twice for a Hep panel and all negative until November 2. He now shows that he has been exposed to Hep B, but not exposed to A & C. My question is can an exposure reading turn into a full blown Hep Virus? Could this have been caused by his immunizations, he hasn't had any immunizations for I believe 3 years.

Thanks-