What to do if I suspect my neighbor has abandoned his dog?

Found a dog in my apartment building’s hallway. He’s super friendly and is waiting outside a particular door. I knock on the door to ask if he’s missing his dog. He claims it’s not his but the dog gets super excited whenever he sees him and only sits outside his door. I’ve posted the dog to next door on the off chance he belongs to someone else but I have a feeling he belongs to my neighbor. I can’t bring him inside my unit because I have cats and an old dog. Not sure what I can or should do at this point. Don’t have a car as my husband took it with him and he’s out of town for work for the week.