I noticed that with my female guppy a while back. Sadly within 2 days she had died. He isn't looking too great, his fins look a bit torn aswell. I don't know wha you can do i'm afraid. I tried melafix and it did nothing. Sorry
He looks a bit knackered to be honest. Does your LFS have a refund policy where you can bring in a sample of tankwater + dead fish and get money back. I know some round where I live do this.
It isn't a disease. So if one of your fish nips it, the healthier fish won't be affected. The bent back happens due to vitamin deficiency in livebearers. Sorry to tell you but that guppy's life-span isn't looking that good