He doesn't look TOO long backed actually, steep croup...a good indication would be no more than a hand width from the point of the hip to the last rib, then he isn't so long backed. His hocks are a little high for a barrel horse, but if e has the heart to doit it, I say if it a few tries and let him tell you what he thinks.
As for poles, hes got to be quick and catty, I think he could doit. I think he could be more athletic, more muscle....Overall he is a beautiful horse though! :)
Try him on the barrels and poles just get to messing around with him, see what he can do, find his speed. Any horse can do it, it's just wether they have the heart to.