Is he QH? Probably. If not, he's likely QH cross.
If you're not worried about his registration papers or being able to register him, it really doesn't matter if he is or isn't. What matters is that you can handle him easily, are comfortable riding him and enjoy him.
His conformation is definitely less than ideal, but that doesn't necessarily mean much of anything if you are planning on using him for basic riding. A lot of QH's are built downhill like he is, most survive soundly for many years with minimal issues. If he vet checks alright - and you're not aiming for a world championship - I would think he'll be fine.