Oh gosh, I'd recommend you don't push at all on his back...it could worsen things.
An arched back and pain can most often mean a slipped disc, and this can be very painful (it's nerve pain, after all). You really need to have this kiddo xray-ed ASAP.
That said, dogs can also have neck issues wherein an xray can be deadly bc of the way they manipulate the neck. Is your kiddo holding his head/neck funny in any way?
Another thing is that it is a good idea to get a full blood panel bc sometimes kidney pain can refer to the back.
Please be very careful, and keep him quiet until you have a very definitive diagnosis.
__________________ ~ A friend told me I was delusional. I nearly fell off my unicorn. ~
°¨¨¨°ºOº°¨¨¨° Ann | Pfeiffer | Marcel Verdel Purcell | Wylie | Artie °¨¨¨°ºOº°¨¨¨° |