Sudden flash of inspiration..
I noticed he was sitting forward when he went, then sitting at the back when he paddled back..
Its windy.
He hasnt got enough weight to keep the ends of the hull in the water.
The water is blowing onto the beach.
If he sits at the back, it will turn towards the rocks as the nose is out,
If he sits at the front, the rear is blown towards the beach, turning the nose towards the wind.
I yelled at him to move forward.
It immediately made the boat turn towards the wind.
Sussed it!
He fidgetted with it for a bit until he worked out how to get it to track straight.
He was delighted with that!