No way. Eight month listing: Way too long, in most cases. When I've sold my personal residences, we've made it 3 month listings, and agreed that if we were pleased with the agent's efforts, we'd extend, if necessary. Threatening to sue: I'd say all trust is gone. You no longer have a viable working relationship. Saying that he can let people through whenever he wants? Wrong. You're the homeowner. It's your property. It's your land, your house. Your agent is only your agent--representing you in a real estate transaction. You could require 48 hours notice if you wanted. (No, I wouldn't advise it. Elvis is right: It will discourage showings.)
Sounds as if the roles have been reversed; your agent sounds as if he thinks he's in charge. He's not. You are. Again: Speak to the managing broker. Detail your grievances. Then get a release so you can find another agent.
Hope that helps.
I don't believe he has a leg to stand on, regarding suing for commission if you refuse showings. (disclaimer: I'm not a lawyer), and I'm not a big fan of 24-hour notice, which will slow your potential showings to a crawl. It is NOT a courtesy for him to notify you... you are the owner of the property and entitled to notification/approval of each and every showing. Have the managing broker set him straight, or release you from their contract.