My mom does pony rides, and we have a variety of "grade" ponies-- none are registered, and only one even remotely resembles a shetland, but at every event there is at least one person who, trying to sound knowledgeable, says "So, are all these Shetland ponies?" and I just have to shake my head. They all have different builds and different heights. People in the US sometimes do't have a good grasp on what is a real Shetland pony.
That being said, most ponies in general are much stronger than you think. Maybe you could share a picture of him, and we'd get a better idea of what he is exactly.