How do i explain to a 7 year old why he can't have sugar/salty foods while on prednisone?

Won't help: The average 7yo remains somewhat caught up with santa & the tooth fairy & at that level any complex reasoning you try is likely a waste of time. At ten, when most don't accept the myths anymore it is because they are developing reasoning skills.At 7, clearly state what you expect of him, give a time frame, consider rewarding compliance. Repeat as necessary. Good luck.