Although ANY walking is good for you, if he is not reaching the fat burning zone of his heart rate (about 65 percent of his maximum heart rate, which is 220 beats per minute MINUS his age) he will not lose a single ounce in fat.
He may be walking too slow, giving just a cardio workout OR walking too fast, thus burning up everything BUT fat.
If the latter is true, less really is more.
In a manual job like being a postie, there is no reason why he should be tubby UNLESS an underlying thyroid or heart problem exists.
Look up heart rates, particularly FAT BURNING ZONE or AEROBIC ZONE.
A heart monitior (very cheap these days) may be the best purchase he ever makes.