"He is going to remember that surprise from his teachers forever!" says Lucas' mom, Heather St. Onge
Credit: Courtesy Katherine C. Cohen/Boston Children s Hospital

Lucas St. Onge was in the hospital recuperating from a life-saving organ transplant surgery and couldn’t make it to his preschool graduation, an event he and his classmates had “practiced for weeks.”

So instead, the 5-year-old’s teachers surprised him with his own personal commencement ceremony – in the Boston Children’s Hospital recovery room!

“They practiced and practiced songs and going across the threshold to get the diploma,” Heather St. Onge, Lucas’ mother, tells PEOPLE. “When he found out he was going to miss it, he was crushed.”

Lucas’ teacher “Miss Laurie” decided to “bring the ceremony to him” on June 3 when she found out her student would be unable to attend.

“The children were busy preparing for our Pre-K graduation when Lucas received the call that he got the transplant,” Laurie Archambault, a staffer at the Hellenic American Academy in Lowell, Massachusetts, tells PEOPLE. ‘We’re just honored to be part of this small sentimental milestone.”

The 5-year-old, who suffers from both short bowel syndrome and severe combined immune deficiency syndrome, underwent the multivisceral organ transplant to get a new liver, intestine, stomach, pancreas and spleen on May 22.

Lucas’ surgeon, Dr. Khashayar Vakili says he would have died without it.

“Lucas only had about 20 or 30-centimeters of his intestines left,” Dr. Vakili tells PEOPLE. “It was a big surgery.”

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Lucas is still in recovery, but grows stronger and “more back to his normal self” every day – and the preschooler still can’t stop talking about his impromptu graduation ceremony.

“He is going to remember that surprise from his teachers forever!” says Heather. “His initial reaction was like ‘Oh my gosh, they really came to see me? This is awesome!’ ”

She adds, “He’s had a rough year, but when his teachers were there for him, everything started looking up.”