He is ok riding in the trailer. His bike with training wheels is too small and he will only push with his feet (not pedal). I think I'll take the training wheels off that and put it on the new bike and see if that helps. Unfortunately, there's no other kids in the neighborhood - it's just us. I do plan on inviting my gf over with her kids who are 6 and 8. A little bigger but it may help. Thanks for the suggestions!