Trying to socialize 9 week old puppy in wagon but he keeps trying to jump out

I have a 9 week old field golden retriever. He’s a very confident puppy so far, does great in cars and with loud sounds and new experiences. However, whenever I take him out on our wagon for a walk or to people watch, all he wants to do is jump out. It feels like I spend half of the time trying to get him safely in the wagon. I do have him strapped with a leash, which worries me he’ll develop leash reactivity. He doesn’t particularly try to jump out when dogs or people pass by, I think he just wants freedom to explore (but sadly he can’t because he’s not fully vaccinated yet). Any recommendations? Am I being paranoid about him developing leash reactivity? Is this just a matter of lack of impulse control?

I do also take him out on a sling and he ends up wanting to jump out of that too.