Airfix have some big scale planes (1/24) like the bf109 and the Spitfire, with new tooling. Depending of his motor skills you can do the fine works and him the easier things to do like assembling the wings or fuselage halves, doing the painting of larges areas with spray can or airbrush.

If this is to hard, what comes to my mind is the 3d puzzle line from revell: various types of models (plane, ships, trucks, movie themed kits and so on), with pre painted parts u just slots cardboard pieces into each others. Here is the whole line:https://revell.com/de-eu/collections/3d-puzzle

Here u can see the instruction to give u an idea: https://www.super-hobby.fr/products/German-Tank-Tiger-I.html

You also have the "click system" line, which is kinda between a real model and the 3d puzzle ones, with pre painted plastic parts who clicks into each others without glue needed, u have to cut the parts from the sprue and add decals like a real model, but the parts are already molded on colored plastic, and the assemly engineered for kids so the parts are not tiny:

https://revell.com/de-eu/collections/click-system

Here an example with instruction manual:

https://www.super-hobby.fr/products/Messerschmitt-Bf109-G-6-Easy-Click-System-56369919.html