Puppy has started to lie down on walks when he sees people and refuses to get up

basically the title, he is a 6 month old GSD/Collie and this has only happened in the last 2 weeks but when I bring him out for a walk alone (not with my husband). if he spots or hears another person he will literally just lie down and refuse to move. I’ve tried moving him out of sight of the person and using redirection with treats, and that works for a few seconds but when he hears so much as their footsteps we’re back to square one. Doesn’t happen when he’s with my husband, and also he does seem to get a little on edge, but that could also be from me trying to move him/him not wanting to move. He's a very social boy, and loves new people, so this is very new behavior for him, and I’m at a loss of what to do when it happens. Any advice would be appreciated