C.C.
While I haven't had to deal with this, I found this one webmd:
After 3 days of treatment, you or your child should begin to get better. A child can usually return to school or daycare after 48 hours of treatment. If you apply the ointment or take the pills exactly as prescribed, most sores will be completely healed in 1 week.
I would assume that after 48 hours you should be ok to go to the store. I would just try and make sure he didn't touch his sores while out and don't let anyone get into close contact with him. Here is the link to webmd if you'd like to read the rest of the info.
http://www.webmd.com/a-to-z-guides/impetigo-overview