Hi OP. I think your post shows the beauty of being able to use photographic evidence. It can't be argued that it was nothing. (I've had this issue with teens) If polite requests are ignored its the way to go. That said, you shouldn't have to resort on car insurance claim level of evidence, to prove to someone that its not fuss about nothing.
There's two things really. Perhaps he's always had a mum cleaning up after him and just doesn't realise because the mess magically vanishes. That's just a question of re-educating him in the idea that you are not, in fact, a House Elf!
Secondly if he'd seen it, said fair enough and cleaned it up, that would be one thing, but his reaction is awful. It doesn't imply he has now seen the light and won't just do it again. Back to the House Elf notion. I've come across this attitude in the past and the offender always thinks its unreasonable telling off. That is a teenage approach and he needs to grow up. Its not a telling off, its a request to do things differently and that is something that would cover more than just cleaning the sink.
You know best how to approach this with him, especially as he has compounded the initial fault with a very resentful grudging attitude a very unattractive trait in a partner (and a potential life partner further down the line?) For me it would be a case of shape up or ship out I'm afraid.. He hasn't passed the first audition. Stand firm OP!