OP, you are mixing up boundaries and curfew/rules.
Boundaries are all to do with the limits you set yourself, the knowledge of where you end and where others begin. They are about perspective.
You have poor boundaries. You have set rules that are too strict because you have allowed your anxieties to run amok.
I agree it is counter productive to demand to know where he is going and who he is with when he goes out. Teens need privacy. This is why he won't have any friends around. He wants his own life, away from your scrutiny.
I also agree it may well have been the 6 pm thing and the other rules that have caused a rift between him and his old friends.
Eventually I got the full story out of DS.
There are several ways of getting a story out of a teenager.
There is the bull at a gate way, asking questions that put the teen on the spot, that conveys to the teen that you think he is naive, leads with his heart, is not mature, and also that you are someone who can't handle teen misadventures, need to have your feelings minded, and are quite fragile. You may get the information, but you will close off or weaken lines of communication.
The other way is to express concern that the friend who self harmed is ok, to ask if there is anything you could do to help, to ask if the teen has access to whatever help he needs, to express gratitude that your DS stayed with their friend and didn't leave him all alone in a wobbly condition. This tells the teen that you have some confidence in him, encourages positive qualities such as not leaving a friend who is in trouble, encourages the teen to step back and see self harm from an adult pov, and most important of all, tells him you are able to handle life in general and gives him confidence in you as an adult.
Maybe the DS took on the bully with the crowbar because he doesn't see you as an adult who can handle things or sees in you someone whose 'grown up reaction' repertoire is limited?