If he's not sitting still and showing signs of stress (whale eye, ears back, tight body, etc) and he's just wanting to explore in one area- sniffing things, looking around, meandering, you might just need to lower expectations.
I have a 1 year old Boston, and when I got her, I was living in a neighborhood where I had grown up and lived for 11 years. When she was 10 months old, I moved to a different neighborhood in the same city, where there was less foot traffic but way more dogs (like 2X the number of dogs, seriously), and in a larger building (with 18 units vs the 3 units in our old one), and with an elevator access apartment with no yard, compared to the ground floor, garden access apartment she'd grown up in. There were also tons of dogs in our building and had been none in the old one, or in either neighboring building.
She was a perfect auto-heel, loose leash, great attention kind of a walker in the old neighborhood, but in the new one couldn't walk an inch without darting, lunging, trying to sniff everything.
We were there for about a month, and three weeks in she got markedly better, though she never got as good as in the old place. Now, we just did a cross country move to an industrial town in California, where we're in a dog-heavy apartment complex with very little around it and right next to a service dog program where there are often many dogs running loose in a big (fenced) field on our walking route. She is OK inside the complex after a week, unless she sees another dog, in which case she gets excited, but is a complete mess outside the complex- again, darting and wanting to sniff everything, and when we go past the group of dogs outside she gets anxious (because they tend to rush the gate and bark at her, we've since changed our walking route).
All that to say: moving will affect dogs, in different ways, and the difference in the new vs old place and neighborhood will make a big different in how they react to things, and they may at times seem like a totally different dog in the new place.
Don't worry too much about it, IMO. Let him take things at his own pace. It doesn't sound like it's a fear thing, more just he has a lot to take in and is young and scatterbrained. You could just make him move through it, if its not fear. Yes, there is some risk of fallout with this, but not the same as if he were afraid (again, assuming its not fear). I would probably just let him do what he wants on walks, at this age. At five months, I'm still focusing on not pulling, stopping when I stop, maintaining some semblance of attention to me (checking in), but most all all learning that walks are FUN, strangers are FUN, and other dogs are FUN. If you only get a block in 10min because he has to sniff every little thing, there are worse things in the world. Eventually, he'll be ready to go for a real walk, and its likely a phase at this age/stage of his life (what with the new move). Don't stress, he's not broken, lol!
