How can I balance this purposefully weak player character?

I’m DMing a 6 person campaign in 5E and one of my players made a Warlock that is dreadfully sick, but is bolstered by the Undying Warlock patron. How can I balance giving him mostly temporary hitpoints as a more flavorful replacement for normal hitpoints? For instance he might only have 4 HP but have 12 temporary hitpoints that cannot be regenerated via healing or hit dice, but may be replenished after a long rest.

Is there a good, balanced way to implement this? Alternatively, is there a balanced homebrew Warlock patron or other mechanic that does something similar?