Look at it from his POV - you're trying to screw him by getting a dev block cheaply.
What end result does he want - leave it empty, joint dev, rent it out & share costs & income or sell it & pay CGT ?
The most likely (& probably fairest) result is to auction it - either you or someone else pays the real market price, and not your hypothetical respective biased valuations.
Hi Keith - *** again the auction suggestion - my question about auctioning it is above, hope you can weigh in on that. ****
End result he wants? - well I've stated all this before but there are a lot of previous posts to scroll through. At first I wanted him and I to develop it together. That was a no-go, he wasn't interested. He was adamant against renting it out. He wanted to put it on the market 'as is'. He wanted the money as soon as possible and did not want to do the work to fix it up and make it worth more. Now the 'end result' he wants is the money he would have gotten if we had done that work already.
But please see the question about auctioning as everyone is suggesting auctioning but no-one is saying how it would get to that point.
Me, I wouldn't have wanted it like this at all but I'm OK to wait it out. I've done all the hard yards so far getting work (after being the main carer of the person who died leaving this situation), giving up my studies, cleaning up the place until I realised how nasty he could get, getting the loan.. while he's been impatiently waiting for the money to fall into his hands and listening to real estate agent appraisals.