While I was building the power generators for the radio at the castle Preston jumped into my T-60 Power Armor. Now I can't get him to join me as a companion, thus I can't command him to get out of the armor.

Please check if this has been reported before, otherwise post it in here so it can get logged:

http://www.gamesas.com/topic/1544248-bug-report-index-pc/

Taking the fusion core should force him to leave the PA. At least that's how I stole the PA of an NPC.

Nice!!! Keep that in mind hey!

I went for the mission where you go onto the Brotherhood of Steels ship (I forget the name, even though the internet is right here lol), and I had my crappy raider armour on. They offer a new suit, so I stipped the Raider armour off & gave it to Paladin Dense. After a while & completing a mission I went back to the ship to get the frame back, etc.

Dense has the raider armour on !!! So technically he doesn't have armour on his frame?? Anyway, I got the raider stuff back & now he's walking around in just a frame heh.