After I completed Dangerous Minds, I switched him out for a different companion. After looking back for him, I could not find him. I sent him to either Starlight Drive In or Greygarden, but I searched both and he wasn't there. He also isn't in his agency. Please help.

This is a common bug with Nick Valentine, i have had it occur to me. Check Piper's Office (Diamond City Newspaper), he is most likely there.