Your 3yrs old songs likely to be suffering from a viral rash
and since it has started with face ,neck and post auricular area it is likely that it will spread downwards in afew hours or day.This rash could be measles
like rash which is a viral rash.
Viral rash would subside in a weeks time but watch out for any cold cough
leadinG to breathing difficulty because sometimes there lead to pneumonia
.At present keep him well hydrated and get him examined by his doctor for confirmation of diagnosis.