I would not worry about him landing on his heel. Many coaches teach landing on the ball of the foot or even flat footed. But the fact is, a couple hundredths of a second after he lands on his heel, the rest of the foot comes down so it’s really not an issue.
Now, if you think he is keeping his weight back too much (which could magnify the landing on the heel), then that is a separate issue. He needs to get his hips going and get into foot strike quicker.
He generally misses low and away. when he is “on” he is almost unhittable, but then he just basically looses it and cannot find it. Alot of people have watched him and said that it is his mechanics, and the only thing I can see is in his landing foot.