Is it possible for him to use his bicycle as transportation to some of his other interests? Saddle time can be gotten in any number of ways; it doesn’t have to be a ride of any significant distance. Lots of small rides add up. Also, even 30 minutes two or three times a week on his bike would be better than just winging it. Fitness is essential, but as with most sports cycling uses specific muscles in a specific fashion. So not only to toughen his butt, but to give his body some good “muscle memory,” encourage him to ride whenever he can. Have a blast with him. He’s at the age where he still will enjoy being with Dad. Trust me, it fades before returning again later in life.