I took DD to the doctors with spots when she was about 1 and he diagnosed chicken pox without really looking at her. Her spots never blistered so I think she actually had a viral rash at the time and not CP.
If your DS has CP and the spots started yesterday then I think they would have blistered by now. Although even with a viral rash he will probably need 48 hours off childcare if it's nursery/childminder.
From my research at the time spots can (amongst other things) turn into CP, viral rash, hand foot and mouth, slapped cheek and check with a glass for meningitis. I looked at quite a lot of photos on NHS website at the time!!