This week brought a birthday wish come true for a young boy battling cancer at Children’s Hospital of Philadelphia, courtesy of Flyers mascot Gritty.

Jack Callahan, 4, captured Philly’s heart last month with a note to Gritty in which the Charlottesville, Va., native explained that he is the mascot’s “biggest fan in the world.”

“I love you, Gritty,” the note read. “I want to be you for Halloween. I want a Gritty cake for my birthday, too.”

For his fourth birthday on Wednesday, Callahan got a lot more than that. Gritty paid the youngster a visit at Philadelphia’s Ronald McDonald House, where the two partied with Gritty gifts and activities, as well as Gritty cake from Whipped Bake Shop. Callahan even wore his Gritty costume for the occasion.

Gritty himself posted photos from the party to Instagram, writing, “When pen pals become real pals,” and wishing Callahan a happy birthday. Callahan’s uncle, Ryan Callahan, also shared photos of the celebration online.

“Today was truly one of the most special days of our lives and we will never forget Jack’s smiles, laughter and pure joy on meeting his favorite mascot, Gritty,” Ryan wrote.

Callahan, his family wrote online, was diagnosed with cancer in February, and began regularly traveling from Virginia to Philadelphia for treatment at CHOP in June. Callahan is undergoing chemotherapy to battle the disease, which a GoFundMe campaign describes as a “malignant yolk sack germ cell tumor that had metastasized to his lungs.”

The GoFundMe is accepting donations to help with Callahan’s treatment, and had raised more than $56,000 of a $60,000 goal as of Thursday afternoon. Supporters can follow Callahan’s progress via the family’s Caring Bridge website.