Foster dog crying almost constantly

We got a foster dog from the shelter to stay warm in the storm and decided to long term foster. He’s 9ish and was in the shelter for five days. He’s well trained so we are assuming he had a family. He’s super sweet, but he cries constantly! We can’t figure out what he needs. We’ve tried toys to give stimulation, calming chews, and verbal reassurance. We covered the windows in case it was contributing and move his crate into the bedroom at night so he knows he’s with us. The only time he doesn’t cry is when he’s outside or asleep. We know he’s trying to tell us something but we just cannot figure out what. Any ideas on what to do or how to provide reassurance?