Exposure to chickenpox. Started asap) ?

I was exposed to chickenpox by my roomate about 2 days ago (he developed the rash and was diagnosed 2 days ago). I haven't been infected with chickenpox earlier in my life, I also have very low immunity to infections and fall sick quite frequently. I don't yet have the rash yet, but I'm having fever (99-100), I'm very fatigued, feeling very weak and feeling generally unwell since last 3 days, my skin is feeling very sensitive and itchy all over the body (but very few small sized red spots yet) since yesterday. My doctor is saying we can't start with antivirals until there is clear evidence (the rash) of chickenpox - instead he has prescribed allegra and paracetamol. What should I do after the exposure and before symptoms showing up? I'm unable do any work because of fever and fatigue. Is there any conclusive blood test for this (so that the treatment can be started asap)?