Keeley, can I ask if your pony is registered with the Dartmoor Pony Society or the Dartmoor Hill Pony Society?
The Dartmoor pony is a specific breed which, as I'm sure you're already aware, looks nothing like your boy. However, the Dartmoor Hill pony is basically any pony that was bred on the moor.
There are several sub-categories of Dartmoor Hill pony, one of which is "coloured", just like yours. When the popularity of Gypsy cobs increased in the UK, coloured stallions were put out on the moor, the owners hoping to cash in on the trend.
Sadly, overbreeding of Hill ponies and a drop in demand has resulted in many being sold for very little money, often destined for the meat trade.
I suspect your pony is therefore part-Gypsy cob and part-anyone's guess!