Have you filled up the bar and drained his health to completion at the same time? There needs to be a little bit of health left from the boss if you beat him to get the good ending. If that happens you will also get another cutscene after you have beaten him
The Healthbar of the boss at the end of the fight should look something like this:
There needs to be a bit of health left but the blue bar needs to be filled, you can achive that by beating the boss as fast as you can.
After that another cutscene should play that starts like this:
For video help, here is a lets play where someone achived it (around 4:50 in the video):
hope this helps!