Appendix QH is a term for a QH that has TB blood and can only be bred and produce more registered QHs if it meets certain performance criteria. An appendix QH that doesn't meet the performance criteria is usually gelded and sold without QH papers. The name means the AQHA will add or "append" certain horses with TB blood to the registry on a conditional basis.
So I understand your confusion, it would be easy to infer that appendix means a QH cross, but the term's a little more narrow and specific than that.
What you have is a STB X QH cross.
Hard to judge a photo from that angle, but he looks to have a very prominent wither and the typical STB head and that his head and neck tie on high to his shoulder. I'm guessing that he has an incredible disposition with that cross, and is perfect for a hop on and go kind of horse.
Post a photo taken directly from the side if you want a more detailed critique.