depending on what he's in for (some have restricted diets), my favorite (and more to the point the favorite of friends in the hospital) is to call a pizza place near the hospital and have a pie delivered (put it and the tip on your card). after living off that nasty grey crap they call food in a hospital, that pizza will make you his best friend ever.
with friends who are closer though, I usually take at least one home-cooked meal per week.