advice for me and my son please! :)

I also think GameMaker is a good idea and recommend looking at that as an intermediate step before UE4.

While it’s awesome for a kid that young to want to make games instead of just play them he needs to have the next step be something he can take. Learning GameMaker will be more complex than what he had before, and it can produce real games. But, it is simpler than UE4 to learn.