Billy Joel‘s longtime creative director Steve Cohen is explaining how the Piano Man’s documentary finally came to fruition … and he’s got a health update on the music legend too.

Steve’s known Billy for over half a century and he’s an executive producer on the new HBO documentary “Billy Joel: And So It Goes” … and he joined us Thursday on “TMZ Live” to take us behind the scenes of the production.

Billy was always reluctant to film a documentary because he felt like he told his story through his music … but Steve tells us what changed to get Billy in front of the camera.

The documentary’s fascinating and there are interviews with Paul McCartney, Pink, Garth Brooks, John Mellencamp, Clive Davis and Bruce Springsteen … heck, even Nas had lots of praise for Billy — which may be a shock to many, but not Steve.

Billy’s love life is also explored … and Steve tells us about a powerful conversation with Billy’s ex, Christie Brinkley … and he says Billy wasn’t comfortable with all the attention their relationship garnered back in the day.

Steve’s been in Billy’s circle for 51 years … and he says the guy’s progressing well from a brain disorder diagnosis … and tells us whether Billy’s ever going to get back onstage.