I've got a couple different avenues for ghoul and they're all just half formed, but!
1. On the eve of the war, Ethan's honeymoon turned into a nightmare when he and his new bride chose to stay at a charming little b&b in Dulvey, Louisiana. Lucas, recent inheritor of the property, decided the young couple would be the perfect victims for all his long suppressed violent fantasies. When the bombs fell, Ethan and Mia were in the bayou, fighting for their lives while Lucas hunted them down. The pair managed to escape, only to later suffer radiation poisoning. Generics, dumb luck, fate; Ethan doesn't know why, but Mia died and he survived. With skin rotting off and hair falling out, he survived.
2. Ethan couldn't believe his luck getting his family a spot in one of the San Antonio vaults. Unable to afford one on his wages, he had entered a contest with Mia, hoping both entering would double their chances. They took aptitude tests, went through several rounds of physicals, and finally got the happy news that Ethan got in, that he could bring his wife. By the time they were being rushed into their new home, Mia was pregnant with their daughter.
Not long after the vault closed, a strange mold began to grow in the ventilation system. The overseer worked hard to have it taken care of, but everything they tried only seemed to make the mold spread faster. Particles in the air infected the residents, growing on their lungs and spreading through their blood to every part of their bodies. It didn't take long for a pattern to emerge; only the contest winners were able to live with the infection while the ones they brought in with them suffered quick onset psychosis, then death.
By the time the vault door opened again, an impossibly long time later, the mold grew to cover almost the entire vault. Only Ethan was left; a strange creature somewhere between a ghoul and sentient mold colony. He escaped the vault and refused to talk about what happened there.
3. In the Lonestar wastes Ethan scavenged for anything he could sell or trade to make enough caps to keep his family housed and fed in Flags, a settlement built from the remains of an old world amusement park. When offered a job that could keep them living well for the rest of their lives, Ethan eagerly accepted. All he had to do was retrieve a set of objects from several old military bases.
Eventually he was able to do so, braving super mutants, feral ghouls, and even the Brotherhood of Steel. Ethan brought all requested items to his contact for the job and expected his payment. Instead he was drugged and used to test a substance created from the very supplies he retrieved.
Ethan doesn't remember much between the tests and waking up decades later surrounded by remains in various stages of decay and rot. Between the cracks and flakes of his skin a strange yellow glow pulses from somewhere inside.