My bet is this is a baby horse with no strength and balance for a rider yet. But do show us his feet. Sometimes things are overlooked. I would keep working him and fitting him up and see if the stumbling gets better over time. Also, turning them out 24 7 over varied hilly terrain can really improve their fitness and foot awareness without having to work them. Horses that live in flat soft pasture never learn to watch their feet and pick them up well as well as are not as generally fit as a horse who lives out and moves around using his whole body every day navigating terrain.