Thanks for placing your query here.
I can appreciate your concerns.
As far as i am aware there is no disease with the name--"fist disease" or "first disease".
Yes there is a disease called fifth disease
(also called roseola infantum
The description of his illness as given here does not fit into fifth disease either.
He need to be seen by a pediatrician
Hope I have answered your query.
If you have any further questions I will be happy to help.
Wish him good health .