Just keep in mind that his ideal anchor and yours may vary. Try to avoid forcing him into using your ideal anchor. It may be right, it may not. The important thing is that it feels right for him and it's REPEATABLE. Here's what I would do and this may greatly vary from others' opinions:
1.) Remove any sight from the bow if one exists.
2.) Ditch any standard bullseye type target and get him shooting into something big without a care in the world for where the arrow is landing. Examples are a berm, dirt mound, hay bale, etc.
3.) Re-explain to him the importance of a repeatable anchor point and ask him to find what feels best for him, then repeat it for a solid week or two of shooting to engrain it in his mind.
4.) Explain to him the importance of follow through. Make sure he is NOT peaking around the string at release and if he does, correct it. Draw, release, keep bow on plane, wait a full second, check for arrow impact. If he struggles with it, literally ask him to say "One Mississippi" out loud after the release and while he's still holding the bow upright before he checks for impact.
5.) Install sight and have him shoot a bunch of arrows. If they group, adjust sight as needed.
6.) Shoot, shoot, shoot.
7.) If the problem reocurrs later, return immediately to step 1.
My thoughts and methods may be a little old school but that's how I learned and it's made a lasting impact on me. I didn't even shoot sights with a compound for the first 2 years of owning a bow and I believe it made me a far better shooter. It also re-sparked my interest in longbow shooting 20 years later.
