A side on picture or a video would help. He does look a little pissed, but that could be anything, it's not necessarily because of you! :)
My biggest concern would be the saddle. A short backed horse often carries weight better than a horse with a long back, but then there's the saddle problem; you can only fit a short saddle on it's back and that probably wont fit a very plus sized rider's bum. If your bum goes over the cantle, you're off balance and putting bad bad pressure on his back in the wrong places.