He died after six days - five years ago on Sunday. Within hours of Thomas' death, his eyes and liver were recovered and sent - along with umbilical cord blood from him and his brother - to researchers.
But that wasn't the end of it for Sarah Gray.
She often wondered - what became of his eyes, his blood, his liver?
The Grays had received a thank-you letter from the Washington regional transplant organization, telling them their son's corneas had been sent to the Schepens Eye Research Institute in Boston, and his liver and the cord blood to Duke University in North Carolina.
Two years later, on a business trip to Boston, Sarah Gray called the eye institute, which is affiliated with Harvard Medical School.
"I donated my son's eyes to your lab," she said on the phone. "Can I come by for a tour?"
The receptionist said she had never had such a request. "I'm not sure who to transfer you to," she said, "but don't hang up!"
The next day, Gray met James Zieske, the institute's senior scientist, who told her "infant eyes are worth their weight in gold," because, being so young, they have great regenerative properties. Thomas' corneas were used in a study that could one day help cure corneal blindness.
Thirteen more studies had cited that study. Gray felt a new emotion: pride.
Before leaving, she bought a Harvard T-shirt for Callum, and decided she was going to go with the whole family to North Carolina, where Thomas' liver and the cord blood had been sent.
Zieske also wrote her: "Your visit helped to remind me that all the eyes we receive are an incredibly generous gift from someone who loved and cared about the person who provided the eyes. I thank you for reminding me of this."
A few months later in 2012, the Grays went to the Duke Center for Human Genetics in Durham, N.C., where even though the twins were identical, scientists found epigenetic differences in their cord blood, research that could one day help prevent Thomas' fatal defect, anencephaly.
Sarah Gray bought Callum a Duke T-shirt.
The couple then drove down to the road to visit Cytonet, a biotech company that had used their baby's liver in a trial to determine the best temperature to freeze liver tissue.
Already in the nonprofit public relations field, Sarah Gray became director of marketing for the American Association of Tissue Banks.
Her mantra has become donate, donate, donate, and not just for transplant, but also for research. Even if nobody asks you - doctors are often uncomfortable when a child is dying - bring it up yourself, she says.
At a conference last summer, by coincidence, Gray learned that the Old Dominion Eye Bank in North Chesterfield, Va., had shipped Thomas' retinas to Philadelphia.
She couldn't believe she'd never known this. She immediately wrote to the researcher at the University of Pennsylvania who used the donation in her efforts to cure retinoblastoma, the most common form of eye cancer in children.
Two days later, Gray got a reply from Arupa Ganguly, who runs the lab and is a genetics professor at the Hospital of the University of Pennsylvania.
"It is almost impossible to obtain normal retina from a child," Ganguly wrote. "The sample from Thomas is extremely precious for us."
Ganguly sent Callum a Penn T-shirt.
They arranged to meet last Monday.
First, Sarah, Ross, and Callum Gray went to the National Disease Research Interchange in Center City, which Sarah Gray calls "the Match.com of science." The interchange connects hospitals that supply organs and tissue with researchers who request it.
"This seems to have brought you a lot of peace and joy," Bill Leinweber, the interchange's president and CEO, told Sarah. "You've been such a strong advocate for research and such an eloquent spokesperson for the value of research."
After a visit there, the Gray family went to Penn to meet Ganguly and tour her lab.
Sarah Gray saw the marbled composition book in which the receipt of retinas was logged on March 30, 2010, the 360th specimen to be received. They became "RES 360," short for Research 360.
"Is this the log book?" she asked. "Oh, my gosh."
Gray ran her index finger over the cursive of Jennifer Yutz, the lab manager who recorded the entry.
"Ross, look at this! Med 360!"
Her husband took a look. Callum, then 4, hugged an inflatable Godzilla as tall as he is, a gift from Ganguly, bouncing it on the lab floor.
"Wow," Sarah Gray continued. "Can I Xerox this?"
"We have a copy for you," Ganguly said.
Penn also gave the Grays a copy of the Fed Ex packing slip confirming arrival, which Sarah Gray said she would "treasure like a war medal."
Thomas' retina tissue is so rare, so precious, Ganguly and her team are still saving some of it for future research. Ganguly's staff led Sarah Gray into the hallway, where a refrigerator, innocuous and ordinary, stood across from student lockers. Yutz unlocked it.
Inside were hundreds of 1.5 milliliter tubes - smaller than cigarette filters.
Yutz pointed to two.
"There it is," Yutz said.
"Oh my gosh!" Gray said. She couldn't touch them. The tubes were frozen at minus-80 degrees centigrade (minus-112 Fahrenheit).
"It's the RNA isolated from the retina tissue," Yutz said.
Call it what you will, that was a piece of Thomas Gray, her son.
Ross Gray has long supported his wife's journey.
"It helped her get over the loss," he said. "It was part of the healing process, seeing that there's still research going on five years after. His life was worthwhile. He's brought a lot of good to the world."
"The way I see it," Sarah Gray said, "our son got into Harvard, Duke, and Penn. He has a job. He is relevant to the world. I only hope my life can be as relevant."