Why is my dog is suddenly scared of going outside?

He's an 8-month-old Border Collie mix and I adopted him last month. We've been taking long walks around the neighborhood ever since, and he always seems to enjoy them except for the buses and/or big trucks that sometimes drive by. On Tuesday I took him for a long walk, but yesterday, he only got about half a mile before sitting/laying down on the sidewalk and refusing to move. I figured he might be tired from the walk the day before so we took the rest of the day off. This morning, however, he didn't want to go outside at all. I still took him out just to see how far we could go but he wouldn't walk more than half a block in any direction, and that was with a lot of hesitation and laying down on the sidewalk.

I've checked his paws for thorns or nails and haven't seen anything, and he's eating and playing normally otherwise. He loves treats but when he gets outside he seems too scared and will ignore them.

For some context, on our walk on Tuesday, we did run into several noisy garbage trucks so we took a lot of roundabout ways to get home. I think all those trucks might have scared him. How can I get him to start walking normally again?