A local guy that seemed to know what he was talking about told me it was
Dicrurus leucophaeus innexus. This goes a little deeper that my level of understanding with subgroups but thought I would add his comment on the thread.
Innexus is the resident breeding subspecies on Hainan, which I've heard is now very rare.
Leucogenis is a migrant subspecies that is widespread in mainland China and is migrating at this time of year (I have heard many reports in HK in the last 10 days, and the wintering bird at my house turned up 2 days ago). I would think that it is very likely that migrants also turn up on Hainan.
Your bird looks very similar to leucogenis, but I am not really sure how to distinguish between innexus and leucogenis. From literature I think innexus should be slightly darker with a less defined cheek patch, but there may be individual variation.