I have a little girl who is 3 1/2 and she is used to riding in the stroller with her parents as well. I have a six seater stroller, so I ended up just letting her ride, but encouraging her to walk when she is ready. A couple of weeks ago she started walking on her own volition and she is soooo proud of herself and doing soooo well! She is not a very physical child and parents have been quite protective, so I just decided to wait until she was ready...but then I have the space in the stroller...LOL. For you, when the other babies are there he will not be able to ride right, so no choice, he has to walk. Lots of positive reinforcement helps of course...with my dcg I praise her up and down every time she walks even a block and she puffs up with pride. Maybe you could have a small reward (like a stamp or sticker) for making it to the park?