The 31-year-old father of two, with another on the way, is at Brigham and Women's Hospital in Boston after an hourslong surgery Tuesday. Doctors are working to stabilize his heart. But his family says that's only a temporary solution, and he most likely is in need of a heart transplant.

"Right now, he needs a new heart," Tracey Ferguson, David's mom, said while holding back tears in her Mendon home.

She says her son suffers from an irregular heartbeat, and his condition started to go downhill during Thanksgiving weekend. Since then, he's fought off a life-threatening blood clot.

He's been transferred to Brigham and Women's Hospital. There, Ferguson says her son met with the hospital's transplant team and learned he would need to get vaccinated against COVID-19 to be eligible for a new heart.

"Brigham told us he has to have the vaccine to accept a heart," Ferguson said. "He is not an anti-vaxx person. He has all of his vaccines, but there are some adverse reactions given his condition, and he is a man, he made his decision."

Ferguson says her son's medical team assured him it would be OK to receive the COVID-19 vaccine.

"I think he was scared and defeated when they told him, made him inactive," she said. "Now, he is making decisions, he is trying to work with them to try and rationalize everything."

A spokesperson for Brigham and Women's told NBC10 Boston it can't comment on individual patients due to federal laws.

"Our Mass General Brigham healthcare system requires several CDC-recommended vaccines, including the COVID-19 vaccine, and lifestyle behaviors for transplant candidates to create both the best chance for a successful operation and to optimize the patient's survival after transplantation, given that their immune system is drastically suppressed," the hospital said in a statement.

Ferguson says her son's family stands by him and is comforted by the support they are receiving. A recent rally outside his hospital attracted dozens of people.

Ferguson says she doesn't know what's next. The family has looked at different hospitals around Boston that have different guidelines, but David is too sick to move right now.

"He said to the doctor, 'Are you really going to let me die over a shot?'" she said. "All of my children, they are all vaccinated, he did his own homework, and made his own decision."