It should be fine as long as your nephews sores are completely covered (preferably with waterproof dressings - you can get a pack from the supermarket) and his nails are elatively short so he can't scratch anyone (which is often how it is passed on!).

DS1 had them about 2 months ago and daycare took him back 2 days after the treatment started. Treatment for him was antibiotic ointment and covering them with waterproof dressings to make sure that the germs could not come in contact with the other kids.