I suspect that it was a combination of nerves and straining to control the weapon.
If he is slow firing for accuracy, here is a stance that I have found helps smaller or weaker shooters to more easily control recoil.
Start with a weaver or modified weaver stance; off-foot forward, body bladed, and a good isotonic two hand grip [gentle push-pull]. Push the weapon hand all the way forward and rotate the elbows in until both are pointed directly at the ground. The weapon arm should now be straight and the elbow should now be gently locked. Lean slightly forward and bring the chin/cheek into contact with the bicep [sort of a cheek weld]. Acquire the sights and target and pull the trigger. Light pressure is all that is required to keep the weapons arm straight, as ergonomic leverage does the work. It effectively turns your arm into a rifle stock. It may feel very strange initially. Try it and see if it works for you. By the way, it works better for men than women due to differing joint structure and musculature. Have fun.
I hope this helps.