My advice is that to make the calories you burned running equal the calories you would burn walking, you will need to walk the same DISTANCE as you were running. Keep with the same course/laps and you should get close to what you were burning before. There is some loss of calorie burn because of basal metabolism (the calories you would burn in that extra time if you just sat around), so you might want to bump up the mileage a bit as well.
Walking vs. Running Calories
Walking Calorie Calculator
Since he is able to run, I suggested he include jogging intervals during his walk to bump up his heart rate. After warming up for 5 minutes, throw in a jog for 1-5 minutes every 10 minutes or so. This is a great way to keep challenging your body to new levels. Pure walkers can also do intervals, with a "walk as fast as you can for 1 block" thrown in every few minutes.
Economy Speed Interval Workout