I'd say QH/Shetland. I'm not sure where you are, but around here welsh, dartmoor, chincoteague are extremely rare if not unheard of. I think the QH would give him the extra size. My bet is Shetland stud crossed with a QH mare. He's a cute sturdy pony for sure! Congrats!
It looks like a STANDARD sized shetland to me. I think everyone is thinking Miniature Shetland. We had one at a farm I worked on as a girl that was about that size. Maybe a Sable Island Pony? I know they were brought in to coal mines a lot, have a few old pictures from my Irish Family that look similar. They are a bit thicker/wide (NOT fat) with a sometimes sour attitude well the "pit ponies" were.