I am so sorry. Please find him an LLMD right away because 100mg a day of Doxy is NOT enough for a new infection. Please read what IILADS has to say about dosaging below.
Increasingly, clinicians recommend that certain drugs used for Lyme disease be given at higher daily doses: for example, 3,000–6,000 mg of amoxicillin, 300–400 mg doxycycline, and 500–600 mg of azithromycin. Some clinicians prescribe antibiotics using blood levels to guide higher doses. Close monitoring of complete blood counts and chemistries are also required with this approach.