I'm happy that you have asked me to assist you with your child and hope that you can benefit from my many years of helping parents like you.
Hi. Can you chat right now?
Since you are still offline let me suggest that this is most certainly a viral rash with his age and the history of a couple of days of fever and now a rash. This time of year we see lots of such viruses: roseola, herpangina, etc., all of which are harmless and generally when the rash appears the fever resolves and they feel better.
For his mouth ulcers you can mix children's benadryly elixir and Maalox in equal portions and give him a little in his mouth so swish around and spit out as that will help dull the pain so he can drink.
In any case, continue the fever reducers and the antibiotic and he should feel much better in the next 24-48 hours.
I'll watch for your return to chat however I'm seeing patient so allow me time to get back online if you will.