He "works" with his wife Alexis as a general goods trader. Most of the time, however, when Alexis is working, her husband is spending time with Tina De Luca. She and their daughter, Erin, are unhappy with the situation. Holt also shows distaste for his situation and directs his disdain towards his wife, asking her to "stop nagging" at him in front of the Sole Survivor.
Though the Vault Dweller's Survival Guide for Fallout 4 claims he can be recruited as a vendor for settlements, he actually has no special shop assigned to him in his NPC file, nor any settler variables set in place. Oddly Tina De Luca, another Vault 81 dweller that can actually be sent to a settlement, has partial variables set in place to act as a specialty vendor.
Between 10 PM and Midnight, Holt and Tina will travel to the supply closet on the first floor of the residential area positioned directly below the 2nd floor restroom next to Holt's residence in Vault 81. They appear to have no dialogue, and nothing else appears to take place.
Even after Tina De Luca is sent to a settlement, Holt will still have the same routine and acts as if Tina is still in the vault.
pcps4xboxone If the player tells Alexis to just break up with him, it may not happen. Instead the two will remain together, and nothing will happen. [verified]
pc If you kill Holt in his bed while no one is around and leave the vault, when you return his body will move to where it would be if he was still alive, such as the shop. And no one will comment on his death.[verified]