My dog won’t go outside anymore

So two years ago I adopted a former stray bulldog/pittie mix (estimated to now be 7 y.o.) from Dallas, TX. He's a very sweet, chill, but very strong-willed guy haha. He's really good in most areas, but is very particular about where and how long he'll walk outside. Unless I have someone with me, either another human or dog, he'll only walk in the area in front of our building (in a city btw). Once he's done, he's done. He sits down and pulls back, even if I pull on his leash. Last night I tried pulling him a bit and his collar came off, and he ran back home naked down the street.

Anyway, this has been our norm for the past two years, and I try to just work around it when I can. But two nights ago a family in our building was celebrating Diwali/Festival of Lights and set off firecrackers/sparklers at the entrance to the apartment, right as we were coming outside for our walk. My poor dog was terrified, ran away and was shaking for quite awhile. Since then, he refuses to walk out the threshold of our apartment and refuses to walk in the lobby of the building. I either have to carry him (which is a struggle, as he weighs 80 pounds!) or push him across the floor from behind. I've been giving him Trazodone which he takes as needed for anxiety, but it doesn't seem to make a difference. Does anyone have any suggestions? Thank you!!