God only knows how most people learn wingshooting. Hopefully they start very young and have the help of someone with suitable fundamentals. I was lucky in that my father had some instructing experience, and that I got the opportunity to shoot frequently and early with others. A lot of people become capable of shooting fairly well without the foggiest notion why or how. So good instruction is the way.
I will send you a PM and put you onto a few fellows in the Twin Cities area that can help. At least two are professional wingshooting instructors and have the training and experience, and another is a gunsmith specializing in stock work for wingshooters.
Traveling to a famous shooting instructor will cost a lot of dough and not give much for long term results if your time with him/her is limited. You want some kind of a simple beginning and a way to groove a fundamentally OK mount before you go to somebody like the fellows at Fieldsport in Traverse City, etc. They are good, really good, but you can get yourself to a decent level cheaper, and probably should before you can understand what they can teach.
I recommend local instruction and use of a skeet field, so the student can go back and practice, involving the instructor occasionally, and possibly finding others with which to shoot and thereby learn (everybody learns in different ways). I strongly recommend avoiding trap if you intend to shoot game, especially upland game such as you describe. Trap encourages aiming behavior rather than gun mounting, moving and pointing. I'd say get a good lesson (about $50 around here), then practice for a few sessions on a skeet field -- get cheap shells, shoot a lot and shoot frequently. Then go back to the instructor for a check. Forget about fit for now. Almost any decent modern gun is close enough to start with unless the student is physically shaped very much away from average. Women are a different fit story. Posture is usually the problem behind gun mounting, and it is a much higher priority to correct, than fit. Most of us never shoot a gun fitted to us, but can learn to shoot very well.
The student should tell the instructor his objective. In your son-in-law's case, it sounds like he wants to shoot game - to hunt. That way the instructor can avoid wasting time on the nuances of a game like skeet, which is a mounted gun sport nowadays. If you are trying to create a decent mount a swing for field use, you are not interested in skeet per se. Make sure your instructor knows that. The reason to use a skeet field and a skeet instructor is because of the variety of shots similar to field circumstances, and a way to graduate from a little gun movement to a lot. Sporting clays is also a possibility, but not a good place to start, I think, because deceptiveness is so much a part of the course design. The skeet field is also very compact and the instructor can set up a shot example quickly -- more instruction in a short time, and more repetition. Sporting clays (gun down) is an excellent practice exercise for hunting to which to graduate once you get most of the skeet shots down (gun down). I do not recommend going to trap unless you are already fundamentally sound, or don't ever care to be. If you shoot trap for hunting practice, shoot it gun down, at least off your shoulder, so you can pick up the target witout a gun in the picture, and exercise a mount and swing.
Best Wishes,
Tony Lowe
