It will take six days' hard riding to return to Hatgal from Mongolia's reindeer people. We are four hours in when I hear Zac yell.

Shorty is in full gallop across the hilltop, pulling hard to the left, while my son hangs at an odd, unnatural angle from his right. Shouts from the Mongolians. A strangled squeak from me. But there's nothing we can do but watch. Zac falls. For 10 sickening metres, he is dragged like a rag doll towards the tree line, head bumping over volcanic rocks so jagged it's impossible to imagine his helmet can withstand them. Then, silently, with what looks like a conscious movement, his foot is free and he's down.

I'm off my horse, and running, and I can't even see if he's moving. When I reach him, Zac is scarlet with pain and rage. "My arm is broken," he says. I'm so relieved he's conscious I lose all grasp of diplomacy. "Why do you think your arm is broken?" I ask.

"Because I can feel the bones," he spits, adding a virulent blast of Anglo-Saxon. And, when we cut his coat off, there's a lump in his upper arm which wasn't there before. A rider races down the valley to fetch a jeep to fetch a doctor. Then a motorbike sets out to fetch fuel for the jeep. We give Zac ibuprofen and he sips fluids. He wants Riesen, but we're out of chocolate.

Four hours after Zac's fall, the jeep returns. The doctor shoots him full of Tramadol, field-splints him with sticks, and we bump over river beds and gullies to her clinic in Tsagaannuur. The nearest x-ray is 12 hours away, as is the nearest road.

I buy Riesen, get Zac settled and hit the phone. By 1.30am, SOS Medica, the only international-standard clinic in Mongolia, has a helicopter ready to fly from Ulan Bator in the morning and our insurers are on side.

Twenty-six hours after the accident, the heli arrives. "Am I the first horse-riding injury of the season?" Zac asks. The doctor nods. "I'm honoured."

Thirty-two hours after the accident, we are in the tender arms of SOS Medica, eating pizza. Zac's arm needs surgery, which can't be done in Mongolia. I email family.

Sixty hours after the accident, our insurers have signed off on an air ambulance to Hong Kong, to arrive at 9am. Weather problems hit. It will arrive at 11am … 12.30pm … 2pm … We get to Hong Kong around midnight, almost five days after Zac's fall. His father will be meeting us at the hospital.
• Theodora blogs at escapeartistes.com. Zac blogs, rather less frequently, at kidventurer.com