While I can't help you out with the Blood Hunter angle on his new character, I do have some advice for "removing" an old character that a player simply isn't enjoying, other than falling on their own sword or just wandering off to go tend an inn: Heroic Death
Years ago I ran a campaign where all the players made a stable of 2-4 characters, mixing and matching them into the storyline as needed for whatever worked best for that particular mission/storyline. One of the players wanted to experiment with a psionicist, something neither he nor I was tremendously familiar with. He's a mature gamer, so I could trust him to not play the class in an attempt to "break" my campaign, it was just something new to try out for size. We talked it over a bit out of game and decided to go for it.
Months later he decides he simply doesn't enjoy the class and would rather play his other 2 heroes. I told him, rather than just inexplicably ride off into the sunset, why don't we give him a memorable sendoff? Now, for inexperienced DMs/players, this might be something you want to work out the details of prior to the session, but we were both veterans of D&D and didn't want it to appear too contrived or rehearsed, so I told him just be patient, play the character for another session or two, and trust that I have something cooked up for him.
It just so happened we were running the Night Below adventure, where heroes descend into an underground aboleth city to defeat the aberrations from capturing and mentally enslaving various communities on the surface. Aboleths are powerful psionic creatures, so it was a party concern when going up against the big bad in the final tower. To counter this, the party liberated a small group of derro slaves that possessed psionic wild talents. This crew, led by the player psionicist, were to attack the aboleth mentally just to tie him up. They knew they would be severely outclassed in a mental battle, but the goal was to simply hold out as long as possible in this psychic duel, while the rest of the party defeated the enemies in the more conventional manner of swords and spells.
Rather than make any rolls, each round of the fight I would describe one or two of the derro collapsing in agony, blood spraying out of their noses, eyes, and ears as they succumbed to the more dominant mind. The player, crafty as he is, saw immediately where I was going with this and kept up the attack. Finally, when all the derro were defeated and the full brunt of the aboleth's awful assault was directed at the psion, I told him (and the rest of the party) that he could feel himself losing this bout, and that he had better break off the psychic contact soon or he would join the derro. Of course the player refused to give in. The next round saw blood streaming from his nose. The rest of the party, having no idea the player no longer wanted to run that psion character, frantically stepped up their efforts to clear the room. On the next and final round, the brave psion's head completely exploded, showering the room with bits of skull and brain tissue, just as the PCs landed the final, killing blow on the aboleth king. No attempts at resurrection magic were made, as the party started to get the gist of what was going on between the player and myself. They would've auto-failed anyway, even if they were made.
My advice is for you to try and build a similar scenario, maybe a "Leonidus" like stand where the retiring character holds out against a superior force, buying just enough time for his companions to escape to safety. Or, if he is known for a particular talent, try to incorporate that into the heroic death. For example, if he's extremely strong, he holds the closing doors of a "crushing wall" trap open long enough for the party to get out of the room (like the Cyclops in Krull, to go way old school!).
