Everyone keeps talking about how small this island is and how hard it would be to just disappear. Although the most obvious or likely explanation is that he either fell and was injured or possibly died as a result of injuries, or that he became suddenly unwell and collapsed somehwere, neither of those things explain why he can't be found. He is unlikely to have fallen in such a way that had him immediately being swept out to sea.
Likewise, if he'd gone off alone intending to take his own life (which I find unlikely, but you just never know) I think he'd have been found by now.
I'm increasingly fearful that he's been kidnapped, or harmed in a bungled robbery or something.