He's very fit and healthy. Not at all overweight. We have always run together and now he's bigger we can do longer runs. Nothing punishing - 5k - 12k is our comfort zone. But very often he gets a stitch and it's so painful he can't run. He finds it very frustrating. We've tried making sure he's hydrated, but not too shortly before the run. We're at a loss. Any advice? About one run in every 3 or 4 is a write-off because he gets a stitch and can't get rid of it.