I agree with mamakay. While I guess its possible, seems highly unlikely this was chickenpox with bumps only on one thigh. I'd be watching him closely for awhile. Could be allergic reaction, bed bugs, scabies, and any number of other things I'm sure.
And yes, children get varicella after getting vaccinated for it frequently. Though its said 'it is a milder case'. My two older children had chickenpox after my ds was vaccinated for it. Only in their case the timeline suggested they actually caught chickenpox from the live shot. Idk, can't prove it.
Oh and the chickenpox bumps usually start on the scalp or torso, then spread to the extremities.