Why, during his dream 4th birthday party with the mascot he loved so much, was he crying?
"I'm so happy," he told them.
There hadn't been a day all year when Jack felt this kind of joy.
He began complaining about leg pain in January, but his parents chalked it up to growing pains until one day at school when it was so bad he was inconsolable. Within days, he was so overwhelmed, he couldn't walk.
At first, his doctor thought it might be a virus, then sent him to an orthopedist, who suggested an MRI. The results revealed a grapefruit-sized mass on Jack's pelvis. That led to a biopsy and CT scan, which showed the disease had already spread to his lungs. The diagnosis was an extragonadal yolk sac germ cell tumor.
Jack's parents first took him to the doctor on a Monday. By Saturday he'd started chemotherapy at the University of Virginia in Charlottesville, about a mile from where the Callahans live.
There was some hope that four rounds of chemo and surgery would be all he'd need, but this particular cancer was rare in children, so there was uncertainty about whether he'd require more treatment. When the chemo didn't work, Jack had to seek more advanced treatment out of state, and because Mike had family in Philadelphia, they went to CHOP.
The Callahans have been staying in the Ronald McDonald House, which provides families a place to live while their children are undergoing treatment, and haven't been back to Virginia much since they went to Philadelphia. Emily had to resign her position as the executive director of a nonprofit trade association for an autism service organization. Mike, a city planner who consults with cities around the country, has had to cut way back on his work.
Jack has undergone two more rounds of chemo and three stem cell transplants, each of which require a 30-day hospital stay. They're hoping he'll be able to leave the hospital in early December and begin radiation at the beginning of the year.
Through it all, Jack has stunned everyone with his energy, among his expected lethargic days.
"There was a point in time where he had your standard IV pole and eight pumps connected to him, and my biggest issue was keeping him from doing flips in the bed," Emily said. "He has days, too, where he's just knocked out but he just keeps on going. And yeah, it's so fitting, to me, that he really loves Gritty because, I mean, this kid has more grit than I have ever seen."
Jack has always loved mascots, first taking a liking to Rameses, the mascot from the University of North Carolina where Mike and Emily were on the swim team. They used to let him watch videos of ACC mascots playing basketball as a reward at the dinner table. And he liked the Phillie Phanatic and Gritty too, but it wasn't until his floor at the hospital had a Gritty-themed, in-house summer camp that he became attached.
"(Jack) just really was like drawn to him and he would say, 'Mom look at his crazy eyes, his googly eyes,'" Emily said.
One of their challenges as parents has been keeping Jack entertained and in high spirits while he undergoes all of his treatments, especially since he is immunocompromised and can't be around other children in the hospital. One day, Emily and Mike asked if he wanted to draw a picture of Gritty and write a letter to him.
Emily sent a picture of the letter in their family group text and Mike's brother Ryan asked if he could post it to Instagram. When it started to go viral, word got to the Flyers. They arranged for Gritty to come to Jack's birthday party at the hospital.
The artist who designed Gritty, Brian Allen, sent him a letter with a one-of-a-kind drawing too. It was one of the many Gritty gifts he received.
Emily and Mike decided not to keep Gritty's appearance a secret so he could be prepared for it, and Jack got to spend almost two weeks excitedly telling anyone he came in contact with about his party.
"It helped him snap out of a time when he was just not feeling too great about whatever was happening," Emily said.
And when the day came, he was ready in his Gritty costume that his parents had pieced together from various sources. He also had a Gritty cake.
Jack sat in a little chair with his costume on, surrounded by family. When Gritty came in, the two hit it off. They sprayed silly string on Jack's grandpa and the security guard, and Gritty let Jack shoot hockey balls at him.
"It was amazing to watch because you watch Jack, he's wearing a mask so he it's hard to hear him talk, and Gritty, he can't talk, and he communicates with his hands and you watch Jack then start to adapt his communication style," Emily said. "He would point at Gritty with what he wanted to do and then he would run over. I said to Mike it's like they're friends, in Jack's mind. Now they are friends."
Members of the team's front office came to the party and offered to set Gritty and Jack up as pen pals. They showed footage from Jack's party on the scoreboard during a game at Wells Fargo Center.
The day after the party, Jack got to wear his Gritty costume again for trick-or-treating in suburban New Jersey with two brother-figure friends he'd met through the University of Virginia swim team after his diagnosis.
The support from the Flyers, family, friends, healthcare professionals, and the people at the Ronald McDonald House has been touching, though overwhelming at times. Emily recalls the steps of their 11-month journey with the meticulousness of someone who, along with Mike, has had to stay strong for Jack under the weight of his stubborn and mystifying illness. But that moment of Gritty bliss was much-needed catharsis for all three Callahans before their fight continued.
"It was the best day," Emily said, her voice cracking with emotion. "It really was. I mean, he's had such a tough year. And them bringing Gritty down to the party, it just made him, for like those couple hours, it really was just all about being happy and having joy."