Hi and welcome.
If you know what gun you like then might be worth putting a wanted ad on here. Often will turn up something that will be a lot cheaper than a retail shop.
The S400 is a great gun and the carbine is smaller and provided your son can get on with it you can't go far wrong. Its accurate and easy to keep clean/service yourself.
You are right about the friendliness of the target shooting fraternity. It astonishes me how helpful everyone is in this community. It was one of the main reasons i took up the sport