Thanks Userofmuchtape&glue,
I think the Bixler is an ideal beginner plane and FLITE TEST has a lot of YouTube videos on that.
My son already builds planes and he understands the servo concept. But I'm only scared to give him the motor (a kid's fingers can be cut off fast).
On the flying field I can fully survey my son, and I'm astonished about his precise flying skills. He already lands on a very narrow road in the fields.
User manuals recommend a beginning age of 14. Therefor I have to hide all the motorized equipment in our house. Kids underestimate the danger and try to work then alone in the basement.
I have DIY models in my mind, but it will come later.