I start with infants so they have enough time to practice our walking outside. I am patient. I let them seat at the beginning when they look tired or they want to. They rest a little bit and they go to walk again. As the time goes they start getting in shape for our walks. Currently my kids (between 18 to 24 months) walk between 45min to 1.25 hrs straight (well, we stop to drink water a few times). Maybe that approach could help with your boy. When I received older kids in the beginning we did not walk that much so I am not sure how it would work for you. But I guess, same as adults they need to get in shape if they are not used to move to much.