If you just got him that bike and he was not riding or exercising before he's doing OK. Be patient and encouraging. Just hang in there. Find out what he likes the most about riding. For example, If he likes sight seeing make plans to go somewhere interesting. Does he have a bike computer? That can inspire people to want to go faster or farther just because they can see what results they get. It gives him an easy way to compare progress if he is interested. It's a good stealth way to motivate someone. Don't say why you are getting it just get it. Even if he is not interested he may notice over time that he is improving his average speed, or distance, or max speed. If he likes gadgets that will go over very well. In fact if he likes gadgets, get him off the bike now while you still have any money left.