Does he care about the NHS or is he just a media attention seeker?

You might argue that, either way, it's none of his business.  The Times reports:

Boris Johnson will seize the floor at a meeting of the cabinet today and demand a £5 billion annual cash injection for the health service beginning next year.

Allies of the foreign secretary say that he has a “track record of winning” and will not relent on demands for a £100-million-a-week Brexit dividend until it is secured.

Britain is expected to keep making contributions to the EU during a two-year transition period until 2021, but Mr Johnson believes that the extra NHS cash should be spent from March next year, when Britain formally leaves the bloc. There is no indication of how the money could be counted as a “dividend” from contributions yet to return.

Some might unkindly suggest that the only thing that Boris cares about is his overweening ambition to  occupy No 10.