I'd go option #3. Buy him a 20 guage that he won't have to worry about upgrading in 4-5 years. The new guns, chokes/loads have made 20 guages effective killing machines for anyone, not just women and kids. Check out the last issue of Turkey & Turkey Hunting and I believe the issue before. They covered quite a bit on this.

A good set up and I bet he could have his 20 guage for years to come hunting turkeys. It will not be money wasted.

Sell the Remington fixed choke. With a new 20 and screw in choke option it will become redundant. That will reduce your cost on a new one.

Like has been said, get him started shooting them properly. To be honest I think blasting them with buckshot to the body is possibily irresponsible. I say possibly because my thought is the wasted meat. I just have this vision of breast meat blasted with buckshot. Idea is we eat what we kill. Again I'm not sure what the result of that shot might be but in any event I'd teach him to jellyhead them.

Good on ya for getting him out there. Sure must be fun. My kids are still too young but I'm looking forward to what you're going through.