1. Show him where he's allowed to run, so he understands that running is fine as long as it's in the right place.

2. Engage him, perhaps by getting him to 'help' you push his pram, or to hold your hand by telling him you might get lost if he doesn't help you.

3. Turn it into a game of catch by shouting "Chase me!" before moving just a few steps in the opposite direction as he (hopefully) runs towards you.