Without too many spoilers, can you tell me if I have to take the Castle to get Preston's perk? He stopped talking to me and just kept asking if I was ready to take the Castle. So I guess I have to?

No, I don't think you can. That's what happens when the companion is a quest giver.

Why don't you just take the Castle?

I will, I just wasn't ready to do that yet but was tired of bringing Preston around (just using him for his perk). I ended up telling him that I was ready to take the Castle, so he left to meet me there...but I'm doing some Railroad stuff with Deacon first. I'll get around to it eventually.