I've been unable to tell him to exit power armor to be able to repair it while he was a companion, the option never comes up when talking to him, I ignored it, later I took the Castle, he left the party, and I later completed the quest Old Guns, suddenly Preston decided he wanted to talk at sanctuary when he did, he just exited his power armor, finished conversion.

I took back the frame and parked it at the power armor construction, then I wanted to recover the destroyed power armor parts from his inventory to repair the frame, and all my other equipment I had dumped on him. However when I try to talk to him he just says some standard line then walks away, I'm unable to trade, or recruit him as companion, since no "conversation wheel" comes up. I suspect he bugged sometime during the Castle quest.

Seems this bug is related or similar issue: http://steamcommunity.com/app/377160/discussions/0/496881136913298034/#c496881136913440652

This usually means he detects status that you have not yet completed a task he has set you to do.

Double check Quest log (data in pipboy) and look if you have a settlement quest from preston.

If so, complete the quest, then you might be able to talk to him after that.

After you exited the PA that Preston was using,are you able to use your pipboy out side the PA?

I have some problems when you use a companions PA.

Link:

http://www.gamesas.com/topic/1544248-bug-report-index-pc/page-18#entry24564586

The bug might related that due that you used Preston PA you got the bug.

Try an early save before you entered Preston PA.

I had this bug too, there was a user on the steamforum with a fix (it had to to with opening console and removing him from some weird faction).

but yes apparently a lot of ppl have this bug.

Yeah I did the removefac 00075D56, and it fixed the issue, dunno if it broken something in a quest chain like not set a flag properly.

Before I did the removefac he would still give me new missions like new settlements to help, but I could not interact with him in anyway, like trade, talk or recruit him as companion. I took all the powerarmor parts back, however I noticed he now has flamer equipped instead of his musket, I suspect that flamer is from when we took back the castle during "the boss" fight.

Yep, the same bug. And thus will not fixed in the next patch. Strange, looked like Bethesda don't test Prston quests at all. I wrote to support, but get back automatically generated message to look for suppoer in forum community.