Arching the back may also be a sign of reflux. Definitely talk to your child nurse about it.
I went to the doctors and the doctor i saw said that because he isn't chucking up huge amounts (he chucks up a little bit but mostly swallows it) and is gaining weight then its of no concern and that he will grow out of it.

I might get a second opinion as if hes in pain i'd like something for it.