This is why when ever I do a job for someone, my contracts are EXTREMELY detailed. I write them out as if its going to be used in court.
I cant tell you how many times people blindly sign my contracts with out even reading it. I have now included a cover page with all my contracts with the words in capitalized big bold underline text to read the entire contract!
This stops 95% of the problems later on... For instance I did a job about 5 years ago, a Sears Kit house, with those huge soffits? On the back of the house, there was a screened in sun room, with custom made wood screens, which in the winter were replaced with storm windows. I explicitly asked more than once: do you want me to bid on painting those screens? 'No' was his answer.
So in the contract I made it a point, to point out that the sun room, screens and storm windows, and doors, were not included in my contract.
Jump ahead 6 weeks... (his job took 6 days, I planned 5, but life happens) I call him up sunday night and told him that we have clean up to do on Monday, and some minor things to fix that I am not happy with and we will be done.. He says 'What about the sun porch? that hasnt been touched? I said you didnt want us to do the sun porch, I asked you iif you wanted me to bid on that part and you said no' He said I thought that was included So I responded ' check the contract, its explicitly excluded per your original instructions'
needless to say he wasnt too happy at this point, the sun porch was an easy 1-2 weeks by itself given the amount of work involved, so it wasnt like I could just slap some paint on it and call it a day... the poiint is people need to read what they are signing, get clarification prior to signing. and if the contract isnt specific enuogh it needs to be incase there is ever a problem.
To the OP,
Write up a NICE polite letter stipulating whaat your not happy with. you dont have to say why. but its courteous if you do, and explain that you wish to terminate this agreement (put a date) and ask him for a job to date invoice.
this does two things: One, it allows him to not get screwed over on his costs, but doesnt make a profit, and two, it gives you a realistic number to compare too. For instance, if the contract is for $5,000 and his costs up to this point is $1800, but you were only planning to pay him $1500, now theres a problem.