When my son was around 10, I bought a young Shetland/mix and trained her myself. She was THEEEE most perfect pony for my son as long as I was within her eye sight
I thought my son was making up stories about her bad behavior, until he went riding with older neighbor kids. They not only corroborated his stories but they brought my son back sitting behind one of them, because the pony had thrown him off - lollol It was a funny story then and still is, because nobody got hurt and I had a quick solution.
I put the pony in the pasture, got my 17 yr old Arab/Saddlebred (that I had raised from the moment his hooves hit the ground), changed the cinch on the pony saddle, and sent my son back out the driveway on that big ole (15.1H is big to a ten year old) Arab/Saddlebred ---- with all the neighbor kids.
An hour later, everyone was back at the barn wearing smiles while my son announced Sonny was now his horse and he was never riding that pony again
The pony got sold and my Arab/Saddlebred was my son's horse until I had to lay that horse to rest with cancer when he was 29.
Meaning, ponies have a whole different nasty mind set than horses for some reason. Buy what I call a smaller horse and others call a really big pony, 13.3H or a tad bigger. As long as you have the experience, a bigger horse (pony) will last your son until he hits six feet tall, regardless of how old he is and there won't be any heartbreak over having to sell the little pony for a bigger one