I agree with much of what has been said..
(1) watch for any anger/temper issues, and emotional immaturity or emotional disregulation in general. How does he react when you have an argument, when things don’t go his way, when he is frustrated?
(2) look at his friends. We all have that ONE friend or two- sure- but are his friends largely mature? in stable relationships or wanting to be? How do they talk about or treat their girlfriends/wives? Kids yet? Usually friends are pretty reflective of the person he is or wants to be.
(3) look at the family very closely. All families have some issues and some are closer than others- but how are the relationships in general, especially with his parents? Communication? Drama? Any estrangement etc? How does he treat his mom? And any substance abuse or mental health issues are going to be a giant red flag here.
(4) Ambition/work ethic/career. Are you ok with these as is? Or expecting a change? Usually what you see here, is how things will be going forward.