On Sunday, Brian Munn will celebrate an especially joyous Father’s Day after he donated part of his liver to save his 9-month-old son, Caleb.

Caleb was born with biliary atresia (BA), inflammation of the liver that causes the bile ducts to be blocked and affects liver function. Three months after he was born, Caleb was still jaundiced— which typically goes away within a few weeks in healthy babies— and his parents, Brian, 30, and Brittany, 27, took him to Montefiore Medical Center in the Bronx, N.Y., where he was diagnosed with end-stage liver disease.

“I guess I was kind of anticipating [the diagnosis], just with the Googling I was doing … but I wasn’t expecting it to be as severe as it was, so it was definitely a shock,” Brittany told FoxNews.com.

BA is rare in babies but one of the more common causes of pediatric liver disease in infants, Dr. Milan Kinkhabwala, chief of the division of transplantation at the Montefiore Einstein Center for Transplantation, told FoxNews.com.

Caleb underwent a Kasei procedure, which replaces the blocked bile ducts with the baby’s own intestine to act like a new duct. He went home to Binghamton, N.Y., but one month later was rushed back to Montefiore during a snowstorm— a five-hour drive— when doctors realized the surgery had failed and he would need a transplant. When the Munns arrived at the hospital, doctors saw that his condition was worse than they originally thought; he was profoundly undernourished, which could lead to neurological and physical side effects.

“I think we got him pretty early … Failure to grow may not be reversible at a certain point,” Kinkhabwala said. “We want to intervene when it’s still possible to get back to normal with a functioning organ.”

Caleb needed a liver transplant immediately, and doctors feared he wouldn’t survive the wait for a new organ.

The family reached out to friends and the community, and applications poured in. While Brittany had the same blood type as her son, she was not yet six months postpartum so she couldn’t be a donor. Brian had a different blood type and didn’t submit an application.

It was only after two weeks that doctors asked why Brian, an engineer in flight control aviation, hadn’t submitted his own application. It turns out, children under age 1 can receive organ transplants from donors with non-matching blood types.

Infants younger than 1 have yet to develop the antibodies necessary to reject a new liver, which means that doctors can perform an incompatible transplant, Kinkhabwala said. The baby must take a rituxan, a special medication to prevent rejection of antibodies in the future, but is otherwise a normal transplant recipient.

“That day, I was ready to hop on the operating table, as soon as they told me,” Brian told FoxNews.com. He was tested for compatibility, and four days later, tests determined that his liver was a perfect match for his son.

“I’ve always heard that you can’t mix [different] blood types because all these issues arise, but for them to say… it’s really not a big deal because he wasn’t over 1 year of age, it took a bit to retrain my mind,” Brittany said.

On March 20, the father and son underwent surgery, with Brian donating a part of his liver. Brian recovered easily, but Caleb had to undergo surgery again the following day to remove a clot that blocked his portal vein. Then, a couple days later, his belly became distended and hard, so he went into surgery again. This time, they found a massive bile leak but were able to fix him, and he finally began to recover.

“We are so thankful that the doctors were so quick, so decisive and 100 percent both times,” Brian said. It was only when Caleb began to improve that doctors told them that they initially feared he wouldn’t make it through the weekend.

On April 13, Caleb finally came home. Because they live three hours from Montefiore, he is under the care of a local pediatrician. He’s on nine medications— down from 15— and will need to be on immunosuppressant medication his whole life.

The family, which also includes Noah, 4, and Elise, 2, hadn’t made big plans for Father’s Day this year— they’re just excited to be all together and not in the hospital.

“[Caleb is] doing really well considering it’s only been three months post-transplant,” Brittany said. “A lot of people see pictures of him now and actually almost forget when you look at him what he’s been through.”