My husband's parents are wanting to claim him as a dependent. Are they able to do this? Can I still file jointly with him? Will this reduce our tax return?

My husband lived with his parents last year (before we were married). He had a full-time job but was also a student. We got married in September. His parents are wanting to claim him as a dependent. Are they able to do this? Can I still file jointly with him? Will this reduce our tax return?