Ok...keep in mind that I am a minimalist (partially because I work in a doctor's office and see people overreact all the time).
If you took your baby in already and they have him on antibiotics, they usually take about 24 to 48 hrs to start working. Until then, all you can do is manage his symptoms. Fever reducer. Keep him hydrated. He will be tired, his body is fighting and needs all of his energy reserves for that fight, so he won't be running around. If his fever won't come down even
with a fever reducer - worry. If he is unresponsive - worry. Try giving him spoonfuls of water or pedialyte to keep him hydrated, like 1 tbsp every 10-20 mins. If he won't take that and he isn't having any wet diapers - worry.
Now don't get me wrong, worrying is not bad. I have my own kids, and truth is even though I know all of this, I worry when my kids are sick. If you have a doctors office that has an on call service, try calling them for advice before you take him in. They can sometimes give you advice and little tips to keep yourself sane. But the ER is for emergencies and if he is already being treated, the ER won't do anything but take up your time and theirs, and potentially expose you to more serious stuff(eww).
Hugs to you and your baby.