My son had low wbc and platelets due to roseola virus. when should the go back to normal after the virus is gone? i need to retest him

About 2 weeks.: In general it takes the bone marrow about 10 days to make white cells, a little less to make platelets. Recovery from viral induced marrow problems therefore usually take about 2 weeks from the time that the infection clears.

I would say 1-2 week: Hhv-6 and hhv-7 are the causes of roseola. Both can cause low WBC and platelets. But there are multiple other causes of this drop. Thus documenting recovery of WBC and platelets - is a critical step.

Not necessary: Viral induced leukopenia and thrombocytopenia is common and not generally dangerous. It happens during and shortly after many common viral illnesses and mostly goes undetected, as most kids wont get a CBC drawn at this time. Generally, it is self limiting and not dangerous, but if you insist on seeing a correction, then you should wait at least 3 weeks to recheck, as it will not correct fast.