Dogs are scared/aggressive towards my 5 month old puppy

Sorry for the long post. Tl;Dr: my puppy desperately wants to play with/meet every dog he encounters but they are often nasty towards him.

I have a 5 month old Labrottie/Rottador/whatever you want to call an F1 Labrador Rottweiler cross.

He's very sweet and incredibly good natured, but he's also very playful and doesn't read other dogs signals very well.

He's a big dog. He's already over 30kg and almost every small to medium dog we meet seem initially scared of him. Lots of smaller ones have instantly snarled and frequently bitten him (not seriously, obviously). Especially walking around busy streets, but even off lead most medium dogs are aggressive/defensive towards him. He has never seemed bothered by this and continues to try to play even when they are obviously nervous.

I get that some dogs are nervous and uncomfortable with playful dogs. I also understand that he is too playful/boisterous (he's just being a puppy who doesn't know how big he is imo) with dogs and that makes them uncomfortable. But surely well socialized dogs that are also playful would engage him. It's only large+ dogs that actually want to play.

He's met one or two grumpy/badly behaved bigger dogs that have gotten funny with him. He's also never reacted or even changed his behaviour to this.

I feel sorry for him as he just wants to play but 80% of dogs snarl and lunge at him. Is this just something that he's going to have to live with as a big dog or is it his (or my) behaviour?

I'm also mildly concerned that at some point he might stand up for himself and bite one back (and we all know the bigger dog gets the blame). He's shown no signs of being bothered by it, literally at all, and certainly no aggression at all! Sometimes the other dog has been picked up and he's still desperate to play with it. I am as sure as anyone can be that he would never attack a dog in a way that is serious or uncontrolled, but I can't be so sure that he won't get fed up with dogs biting him for no reason and nip back one time.

Why are they doing this? He loves other dogs and he is the only dog in our house. We have other dogs in the wider family that he spends plenty of time with and is very good friends with, but perhaps only once a week at most.

Is this just poorly socialized dogs? Bad owners? Or something he is doing?