I guess I don't understand the problem w/ having a multi tool or knife. Even for your sons age of 18-20 years old. It's not illegal in almost any situation, there are laws of course, to carry one.

Tell him to keep it in his pocket or bag until someone asks for assistance w/ something or requires him to employ it as an expediency.

As for my life, If someone doesn't like the fact that I carry (multiple) knives that's their problem and to a point they are welcome to keep it to themselves. We can agree to disagree.

Doug has many threads on here for knife rights and responsibilities. Maybe have your son read some of these to cover any conversation he may have in the future.