It sounds like the situation has grown out of proportion. What to do now? You're probably going to have to kiss his ass and be glad to see the ass end of him. I'm sure others on the set would also be happy to see him gone. Work out what you have to do to expedite his exit. It's a little hard to know the absolutely right move to do as every person is different and every situation is different, though I've met a few people who are like that.
Once it's complete you can consider whether it's worth taking him to court. If he's an actor, he may pretty much be judgement proof (when they are so poor, you hope to only make a point in court).
I'd check with a solicitor. He may be attempting to put you under duress as to get you to agree to extra terms. I'd check with your solicitor whether if you made a new agreement under those conditions whether he'd be able to enforce it.
I'd avoid voice overs if at all possible, even to the point where you bring in another actor do dub over his lines.
What you should have done is fire his ass as soon as you saw the writing on the wall.