I suspect he was just hilltopping on a very safe pony.
On of my trainer's students is a small person and started taking lessons on her own pony when she was about 4. She wore a safety vest because every time the pony shook, she would fall off. If the pony took a misstep, she would fall off. Does not matter how safe - still to dangerous, in my opinion.
Our daughter made her leadline debut at 19 mos. A month from her 3rd birthday, she came to Thanksgiving hunt in her teensie attire, well appointed & too cute for words. She sat on a horse for photo opps and then departed safe & sound with her grandparents. I cannot imagine her having gone out with us even if she had gone w/the hilltoppers.