He could be a Stock horse, to me it looks like he has some TB in him, although doubt he would be full as most full TBs here are ex racers. If I had to pick a breed I would probably say Australia Stock Horse.
He could just be a random mix breed, we have a fair few of them here. My first horse was like that, looked a bit stocky, a bit QH, a bit araby and a bit TB, with maybe some pony. If he's unregistered then chances are he is a mix, because if people are going to go the effort of breeding a specific breed they usually register them. I think mixed breeds can be more fun though.
I like your horse though. He looks smart and kind to me.