When did your apartment dog start liking walks?

I have a 4 month old golden retriever/spaniel mix who isn't big on walks. We live in an apartment, so besides his grass patch on our balcony, going on a walk around the building is where he's getting most of his outside time during the week.

I know he doesn't need long walks at this age, but I want to get him comfortable being outside, start taking some potty breaks outside etc.

He's much better walking when it's both my husband and I - but with just one of us he doesn't want to leave the apartment and will sit & whine at the door (or if we do get outside, he begs to go inside any door we see). We're doing "treat checkpoints" and rewarding him at several points of the walk to create a positive association with being outside. He's never had a negative experience outside, he just seems to hate it and can't wait to get back inside.

I've been taking him daily for about 3 weeks now since our vet cleared him to go on walks and not seeing a ton of improvement, although now he will sniff a little bit whereas before he wasn't interested. Sometimes he seems to get scared shitless (literally) and will have runny poops on our walks (whereas on his grass patch his poops are perfectly hard & normal) - I feel bad about this but not sure what to do about it!

Our apartment sits on a walking trail where other people take their dogs, and we're in somewhat of a city area so there are noises - but he hears the same noises sitting on our balcony with no issues so I'm not sure what he's scared of or how to help him.

If anyone had this experience with their puppy - when did it improve or was there anything specific you did to help them enjoy walks?!