So I told him I'd join the BS and have done a few quests for the scribes and a couple for him trying to find what happened to the lost team (going to National Guard station now). So when can I ask him to join me as a companion?

You have to progress through the story, later the brotherhood will arrive in a large way and after you do the first quest after they arrive, he will become available.

After the Shadow of Steel Quest.

You should get him shortly after that.

You must progress through the MQ until you complete the quest called "Reunions". Then the BoS questline continues (or, more exactly, it starts in earnest) and few small quests later he will become available.

Well dang. Was hoping to start using him. In the MQ I'm still at the point where I need to find the detective.

Should I even be doing these quests from the Police Station? Do they even matter or are they just another way to get experience?

Do a few, get the quest to find a missing patrol and then you don't need to bother with it anymore.

They're radiant so the latter