I guess it is more important to tell him how you feel about it, then leave the choice up to him. After that whatever he chooses be there for him as support. That seems like a big goal but then again maybe not. It really comes down to how motivated the person is. As far as the fact he has never completed a spin class is not a true marker of a person's cycling fitness. I had ridden a century and many shorter long rides, but when it came to my first spin class I didn't make it. Spin instructors have got to admit they put a lot of worthless exercises in classes that don't carry over to what people do on a bike. Sometimes the only similarity is the fact you are pedaling.