Tomcat
Question: Has he ever shot a shotgun or centerfire rifle before?
First I'm sorry to hear about your experience. The choice of choke doesn't affect recoil or muzzle blast. Use the full choke for Turkey hunting and, if it patterns well, go with it. If not, get an extra full choke, either from Mossberg or an another manufacturer. The choice of a ported choke is more difficult in your case, as it sounds like your son is frightened by the muzzle blast and not the recoil. Ported chokes are louder because they disperse the gas at right angles from the muzzle, thereby reducing recoil but increasing muzzle blast.
My son (age 10) doesn't have a problem shooting rifles or shotguns,so I can't offer advice based on an adverse experience with him. He killed his first deer at age 8 with my 30.06, and since has takentwo more with it. I got him the 870 youth gun with a Primos Tightwad Ported choke. It is loud, especially with the 21 inch barrel,and it kicks a bit, but not excessively (that is a personalopinion however). Hearing protection is a must when target shooting.
My suggestions:
Absolutely use hearing protection everytime he shoots it other than while turkey hunting.
Let him take his first shots with light-load, low brass dove shells from a standing position (you can show him that the gun doesn'tkick with these shells by shooting it yourself). From a standing position it is easy to roll with the recoil. Throw some stuff up in the air like dirtclods or cansfor him to shoot and make it easy for him to have some success and positive feedback. You might also consider stationary targets like plastic 2-liter bottles full of water as someone mentioned.
Move up to heavy loads or turkey loads, again shooting from a standing position.
Only after he is confident would you let him shoot from a sitting position with the gun, aiming at turkey targets at 20-25 yards. Make it easy for him to hit the target squarely by using a rest of some sort or shooting sticks.
Consider purchasing a PAST recoil pad for him to use. It slips over the shoulder and is a great recoil reducer. Itmay give him confidence that he can shoot the gun without excessive recoil. I use it every time I shoot muzzloaders, centerfire rifles, or shotguns at the range. I even use it waterfowl hunting when a lot of shooting is expected. He could wear it under his clothes for turkey hunting.
Here is a link to the PAST recoil pad:
http://www.battenfeldtechnologies.com/350010.html
You will have to take some baby steps to introduce him to the gun and establish his confidence. Take your time and let him have some fun.
Good luck
Mouthcaller