Hi Andy B,
I’m sorry to hear of your experience with this web developer. As Aunty B highlighted, your choices will depend largely on the specifics of the agreement you have with the developer, but that doesn’t mean that you’re entirely locked in.
That is, while the IP related to the content management system or any custom code could be owned by the developer, in most cases you will own the design and content elements (if stipulated in the T&Cs) and it may be that a replacement developer can piece together the site very simply using those foundation elements.
It may even be that your developer is using free, open source systems on which to base the site. If that’s the case, even the developer doesn’t own the IP for the back-end of the website and it should be easy to port things across to another supplier; perhaps by negotiating a signoff on half of the project, whereby the developer hands over everything in its messy, incomplete state, in exchange for the 50% you’ve already paid.
In any case, it’s worth looking through your terms and arranging for a meeting to discuss the situation with your developers and to part ways amicably, with them pocketing 50% for the partial job and you gaining the ability to have another, more responsive, developer complete the project in exchange for the remaining 50%.
I hope this helps and wish you the best of luck in resolving the situation.