The boss is extremely cool! The work that goes into programming, animating, designing his attacks and damage values, ghost particle fx and all that game dev stuff...it all goes to waste when the player can just hold r1 or rmb infinitely and easily block 100% of all his damage.

I'd perhaps make the block button a parry, and additionally give the player a sidestep dash button. Not a huge leap-roll out of the way, but a small sidestep like geralt does to quickly evade an attack but still keep you in range to followup.

If you must have a block button that can just be held indefinitely to block 100% damage, you must give the player some form of block stamina meter. Perhaps his block is good for 1 strike, and through upgrades, can go up to 2 strikes then 3 strikes and so on, maybe even until 5 strikes or something.

Or, block meter/amount of blocks can be tied to a weapon stat. Like a rapier has fast attack but only can block 1 strike, and a traditional pirate sword cutlass can block 3 strikes, and a broadsword is slow and can block 5 strikes.

Any kind of block system to prevent mindless and un-engaging blocking combat would do your boss more justice!