He looks like my horsey that I "saved" not to long ago. Mine is an appendix, but registered with AQHA. I would def say he is appendix.
He does not look that skinny, That may just be his typing. Once you find out his breed it will help. Mine is about 16 hands and tall and skinny. He's very "huntery" How old is he? Most young horses go through that "lanky" stage.