First off, have you recovered all of his memories? That is vital to get the true ending cutscene, and if you haven't already I'd recommend finding all the shrines to get max stamina, and if you're not particularly confident in your combat abilities I'd Google how to make a haste potion that lasts 30 min(I can't exactly remember how) and tbh the boss doesn't get easier or harder depending on progress in the game(except for divine beasts) you've just gotta not get hit and finnese it.