Well, this is not unusual. Each client has their own reasons why they hire a personal trainer. Your job is to educate them as much as you can and be the motivator they really need. If this client just wants a walking buddy then for now this is all you can provide for him. You can try incorporating a few bodyweight exercises along the way (while walking) in order to add some extra movements into this routine and spike things up a bit. This way he’ll still feel that he’s getting what he wants out of his time with you and also a chance for you to provide some extra benefits to his training. I hope this helps.