allergic reaction to antibiotics

Hi
My son of 11 months recieved antobiotics for an ear infection.
He has broken out in rash all over his body ,3 days after taking the antibiotics.
The rash is getting worse.

I know that kids can have an allergic reaction to antibiotics.
I would like to know how long the reaction lasts for and wht can be done to treat the rash

I am thanking you in advance for a fast reply as always