It's a stroller not a car seat so if you are just going for walks stick some rolled up blankets under and/or around him, I'm sure it will be fine. If it isn't, go for a walk with him strapped to you until he is ok in it.
I’d find one that can lay flat.
The reason they recommend 6 months and up is because of their ability to hold their head up and sit better by 6 months. Not all babies are able to before that time but generally around 6 months they are able to. I put my then 4 month old in a 6 months plus seat because she had good head control and I could tighten the straps enough to where she wouldn’t just flop around. But it felt like she kinda leaned a lot so I waited a bit longer to use it as a primary stroller and kept using her Doona a bit longer.