Ok, so contrary to what a lot of people are saying. The NPC's can actually still take your power armour even if there is no core in it. In my game Nick Valentine was left at Red Rocket where he has randomly aquired a Fusion Core and placed in the armour all by himself. My cores are all stored in chests and are not in the work bench by the way.

SO! I've found a way to get the annoying bastard out of the power armour!

Just take him on a job with you and ask him to unlock any terminal. He will exit the suit and you can pinch it back.

Give him a slap and be on your way.

Hope this helps!