Chow's are notorious for the lack of expression. It's what makes them good hunters and protectors.

Perhaps this poor guy was never given attention by his owner and the only attention he received was from visitors.

As everyone has said time will heal. Treats and hand feeding are usually the best way to bond. I know it's a big no-no but I have went as far to let a feral cat eat off my plate in order to form a bond.