Whaat can I do about him, he is clearly dishonest
Tell him that the contract was clearly wrong and therefore void. Tell him that either he repays to you all money you have given him as a deposit and whatever is left for the payment period after you move out and no longer use the flat or four things will happen:
1) you will report him to the public prosecutor for the crime of fraud (he claimed the flat was 50% larger than it actually is);
2) you will report him to the tax office for non-payment of taxes (he won't be declaring the income he gets from renting that flat);
3) you will report him to the building administration (he'll need permission from the community to rent the flat out and will pay higher charges, possibly he even doesn't actually own the flat but just has the right to live there and cannot actually sublet the flat);
4) you'll file a court case for return of your money and your costs.
He'll pay up.
And find yourself somewhere new to live.