Translation:The suit is too small for him but in a year or two it is going to be just right.
I think the implied meaning is that "he" in question is too fat for the suit currently, but after a year or two of a good weight-loss regimen, it'll fit just fine. What's more, people do shrink as time goes on. Most elderly folks are not the same height they were when they were younger. It could be either, but it most likely the former.
This is a different sentence from the one beginning "Костюм ему немного велик..." in which "a little large" is an accurate translation.
Isn't мал simply a predicate adjective meaning "small" and there's nothing in the sentence suggesting "a little small". That would be немного маленький, wouldn't it?
Or is it just an idiom?