How come so serious ga. You said his mouth get inflammation. It is because of 發炎, or too 熱氣 after talking the medicine? For the 1st case, you gonna to consult the doctor again immediately.
If it is the 2nd case, you can give some mild 涼茶. Last month, my BB also got a cold. After he took the medicine, he did not have 臭臭 more than 4 days. So I gave him 金筍竹庶馬蹄水 (no suger) for him. He like it very much as it taste good. And then he became much better on the other day. You may try. It is very mild, you can give him 3-4oz or a little more if he want. It's not 中藥, so don't worry it will affec the 西藥 he's taking.
When he's sick, he don't want milk or congee anymore. It's okay for few days. But he must to drink a lot of water as it helps him get better quicker.