No idea what your situation is but unless you have asked him to take down the existing ceiling why is he not just plastering it. Get more quotes from more plasterers, they are free. Never never never put all your eggs in one basket make sure you have spoken to at least 3 contractors to get a full idea of whjat is involved in each job. Its unlikely they are out to rip you off but each may have a different interpretation of the regulations and by putting your best interests first it may be that you require a new ceiling as its cheaper to do it now that later when its been plastered. Please get all of the info your house is the most expensive investment you will ever make.