My 20month Old Son Has A Light Pink Rash -

My 20month old son has a light pink rash - tiny pin prick spots that are raised. They started around his nappy area & then spread to cover his tummy, back, face, neck & a smaller amount on his arms & legs. His skin has gone very dry, especially his face. He has a clear, runny nose, no fever, but it abit cranky & irritable. Any ideas what this could be? I'm mostly concerned about him being contagious. Some more info - The Rash appeared about 48hrs ago, he has just finished a course of antibiotics for tonsillitis & ear infection - is it possibly a post viral rash??? Thank you.