From what I've read in cat books and seen on big cat documentaries, felines have a very limited aerobic ability, which means that they get puffed out very quickly (something to do with their heart/lung capacity). They're designed for short sprints, so if their prey keeps running, the cat loses it by being outrun.
Cats can only exercise for short periods, then need to take a break before continuing.
This may be what's happening with your cat, Helen; but it's probably best to have him checked out to be certain.