I think the first thing to do is to take him to the vet if you haven't already. It's not really the rescue's fault if they rehome a kitten they believe is healthy, only for the kitten to develop health problems later.

The point where I'd be saying contact the rescue is if they knowingly rehomed a sick pet to someone, for example my mum took a elderly cat on from a rescue and he was known before adoption to have problems with his back legs, tests and scans showed his spine was crushing itself and from the severity of it the rescue Mum got him from knew something was wrong with his back legs but rehomed him without finding out what the problem actually was.