I think you’re looking for an open source game that is small, can be built and run, and your son likes already. From there he can start tinkering with code and

I think you’re looking for an open source game that is small, can be built and run, and your son likes already. From there he can start tinkering with code and assets. That way your son directly sees the results of his work. Creating an entire game from scratch even with the best tools might be too much in the beginning and he will lose interest.