Thank you Jerry. On page 2 I posted a 3D model, the driveline will look like that and will be mounted to the frame. Above, on the pictures, you can see a one-way bearing with normal bike size #25 pitch, this one-way bearing sprocket will drive the sprocket which is between the cranks. The rest of the driveline - to the wheel - will be like a normal bike. (It's hard to explain, especially because my English isn't that good. I hope you understand it, if not, just ask! :))

To be honest, I didn't expect it to be so expensive. Small things like the 2 one-way bearings costed me $160, which is quite expensive for some bearings imo. Just costs I overlooked... A rough estimation: €1500 (about $2100) including everything except the bike itself.

Great to hear your son wants to do something like this. Sometimes it's really frustrating for me, I never did something like this before (I'm 17 myself) so it's very new for me too. But everytime I finish something (painted the bike, made the driveline etc.) I get a huge smile on my face which is awesome and keeps me motivated!