(CNN) Kieran Shafritz de Zoysa, an American 11-year-old spending the school year in Sri Lanka, dreamed of being a neuroscientist and working on solutions to diseases, his father says.

The boy from the Washington, D.C., area was with his mother at a hotel for a breakfast buffet when he was killed in Sunday's string of suicide bombings on the South Asian island, his family told Action News 13 in Las Vegas

Kieran's Washington school, the Sidwell Friends School , informed his schoolmates' parents of his death in a letter on Monday.

"(The attackers) do not know what they took from the world. They took a great mind, who was going to be a neuroscientist and who was going to work on Alzheimer's disease," his father Alex Arrow, visiting relatives in Las Vegas on Monday, told the Las Vegas TV station.

Kieran was killed in an explosion at the Cinnamon Grand hotel in Colombo, where he and his mother were intending to eat breakfast at a buffet, Arrow told the station. Hundreds were killed in terror attacks at multiple locations in Sri Lanka on Sunday morning, including three churches and three hotels.

Four days after the coordinated attacks, the Sri Lankan Health Ministry revised the death toll, saying 253 people had died. That toll is significantly lower than the 359 initially reported to CNN by a Colombo police spokesman this week. The health ministry cited the condition of remains and the difficulty in identifying them for the discrepancy.

Arrow said he'd tried to call his son minutes before the explosion.

"He texted me back, 'Can't talk right now,' because he was in the elevator, going to breakfast," Arrow said.

Arrow said he'd just spent time with his son a week earlier in San Diego, where the boy spent spring break.

"His mom told me that on the plane ride back to Sri Lanka, last week, he said that we wanted to come back and do more kayaking," Arrow told the station.

"Kieran was articulate, insightful. But he made everyone that he met want to be a better person," Arrow said.

Though Kieran was studying in Sri Lanka this year, he otherwise lived in Washington with his mother, their neighbor Nora Maccoby told CNN.

"He was so sweet -- very, very sweet," Maccoby said.

Maccoby's son and Kieran were friends. Kieran was good at karate, and a peacemaker, she said.

"If there was any conflict, he would laugh. He laughed a lot."

Kieran was on a leave of absence from Washington's prestigious Sidwell Friends School, whose students have included presidential daughters Malia and Sasha Obama and Chelsea Clinton. He was due to return for middle school in the coming school year, principal Mamadou Guèye wrote in an email to parents of Sidwell Friends students.

"Kieran was passionate about learning, he adored his friends, and was incredibly excited about returning to Sidwell Friends in the coming school year," Guèye wrote in the email.

Update: This story has been updated to reflect the death toll has been revised by the Sri Lankan Health Ministry.