Hey there! Thanks for leaving a comment :)
You can't just hit the boss. He has a health bar indicating that it doesn't take any damage when you just hit it, also the hit sound is different. But he has a weak spot which you need to expose in order to damage him.
SPOILERS BELOW
If you lure him towards the edge of the arena so he misses you when he charges, he'll get stuck and expose his vulnerable back. Hit him there and you'll see his health bar actually declining :)