I know that prednisone will dramatically reduce the immune system, so he must be kept away from anyone that may be sick or around someone who was sick. My mother and grandfather are both on it. I had just a tiny touch of the flu a week before I saw my mother. A week later I thought everything would be fine. She turned up with the flu about a day later and it was awful for her. The same thing happened with a sinus infection. She caught it from me somehow after a visit and it was full blown in her a couple days later. My grandfather caught a cold from my son, and they never touched each other. My son had a slight runny nose. My grandfather had a terrible cold and I think it turned into bronchitis.
It causes them to gain water weight, gain fat weight very very very easily, and to look very round faced. The face gains water weight and it looks really puffy, swollen and a bit red. They gain weight very easily, so the diet must be strictly controlled: no sugar, no salt, no matter what. Any weight gained is impossible to lose when on the prednisone. You have to lose it after getting off the prednisone. It's difficult for an adult to manage, so I suggest seeing if there's a different drug to try before prednisone.
Or... maybe they can keep the dosage very low and for short periods of time. Then give the child's body to regulate back to normal and try the meds again.