This can alarm parents when one of their children desires a lock on his door. It is not necessarily a bad thing for a young man to want privacy. And for some, a lock on the door is not in the best interest of the son. The real question is your son trustworthy? And if he is, do you trust him?
There are obvious reasons to not trust a person; a history of illegal activity, drug/alcohol usage, self-harm history or threats to do so, sneaking friends into the house without your knowledge and a history of abusing children. If any of these are present or have been part of a regular pattern in the past in your son, then a lock is not wise. But there must be a history of these things or things like them to warrant such distrust.
To not trust your son, again, if he is trustworthy, is a sign that work must be done, but not on the son’s part. The issue is the inability to trust and a good therapist can be of great assistance in finding and resolving the issue(s) in the parent.
As a person matures, the ability to establish one’s own boundaries is a huge stepping stone to adulthood. As a child grows it falls to the parents to place boundaries in the child’s life to protect them from harm. But as the child grows older, the need to set up their own boundaries is a skill that has to be learned, and this is often by trial and error. As the son or daughter goes through this process, it is best for parents to get out of the way and drop “some” of the protection as they progress into boundary building for themselves.
Remember, the task of parents is to prepare the next generation for life in the adult world. Will there be failures and setbacks? Most likely, but the best life lessons usually come through failure, not the success. But by the time they are ready to go off to college or set up a household of their own, they will need the skill of building their own boundaries.