(I'm not a doctor - just someone with personal experience).

Blisters sound bacterial. However he may have a viral infection (which cannot be treated, obviously) and be getting bacterial infection on top. If at the end of the current course of antib's his symptoms are not improved --sufficiently--, make sure his doctor knows this, because antib's can do a harm as well as good in someone with a viral infection (trust me, I know from experience!). Was his blood tested for the mono virus infection? Tell them about the weight loss too. Good luck.