It doesn't sound like he'd be gaining ANYTHING beneficial from running more than two days a week. And walking at a lower pace would be better to keep the muscle, too.
And you gotta EAT to make that muscle stick, man! Tons of carbs, tons of calories, tons of protein, hell don't be afraid of a good bit of FAT too. With your size and cardio regime, you need all the food you can get. In fact I'd say 1.5-2g of protein per lb of bodyweight for starters, see how that goes.