The shrapnel wound in Aaron Hern’s left thigh is healing.

He has thick staples down his leg, but the athletic kid from Martinez, Calif., has been up and about in a wheelchair and on crutches. Doctors have promised a full recovery for a boy who plays football, baseball and basketball and runs a sub-six-minute mile.

It’s what’s going on inside that worries his mother.

Aaron happened to be standing at the finish line of the Boston Marathon next to 8-year-old Martin Richard. The boys didn’t know each other. But the same explosion that shredded Aaron’s leg tore through Martin and killed him. For several terrifying minutes, Aaron lay alone on the sidewalk, staring at the small boy.

“You can’t take it away,” said his mother, Katherine. But she added, “We’re the lucky ones, and I’m beyond grateful for that.”

If all goes well, the family will return home about May 1, the day Aaron turns 12.

Brigid Schulte