I lost him for a while, had no idea how, and I cant remember when I lost him. BUT have you checked Memory Den in Goodneighbour? Go have a gander
I just hope you haven't done a certain mission yet that leads you there.
I sent Nick to GreyGarden once, and he got trapped on the overpass above.
You checked the last settlement that you specifically sent him to before you had him come with you? Either that or maybe go to where you think you sent him and sleep for a few in game days and maybe he'll eventually show up.
Have you tried turning it off and turning it back on? jk.... No, I don't believe it's related to that quest. It's not one of those quests where he needs to be somewhere specifically to trigger an event. I found all the tapes while he was sitting his lazy robotic tail at the Castle. I think I already had most of them before I even initiated the quest just from thoroughly looting every place I went to.
Push comes to shove, you may have to just go to where you dismissed him at the tower and dismiss your current companion to Sanctuary and just follow them. Maybe he's stuck somewhere along the path?
Have you checked his agency in Diamond City? I experienced a weird bug with Curie the other day (never had issues with my companions before - I'm on my 3rd play-through on PC).
I sent Curie to Hangman's Alley and she never showed up. I found here again in Vault 81 in front of the doctor's office. I took her with me to Hangman's alley and dismissed her while we were there. I then slept for 6 hours and she's gone again.
I guess she doesn't like the place? LOL I build a nice home there for my companions and it has plenty of defenses. *shrug* I haven't looked for her since then, but I guess she's back at vault 81...
If you are playing on a PC (and if not, my apologies for the suggestion), there is a console command to transport your character to the current location of a chosen NPC. That's how I found Nick that one time.
Awesome! I'm glad you found him again! Nick is a cool guy, would have been sad to just leave him there! But yea, I also think that Companions should just walk out of sight and then teleport to where you send them. Makes no sense to have them walk all the way. I actually go through great length to teleport to the place I want them at, and then dismiss them there. LOL