Your little one is adorable
I think the "bump" on his back that you speak of is just what is known as a "roached" back. My Luna's back is like that too. So far it hasn't caused her any issues, it's just a fault that would make them unsuitable for showing, along with the coat colors being a bit off. Luna came from a pet store, and is pretty poorly bred, crooked teeth/bite, the roached back, silvery colored coat, etc. but that doesn't make her any less loveable
Your baby appears to have very long back legs, which could be why his back is curved like that. A vet checkup would be very smart, just to get a baseline on his health, and to make sure that he is in good condition.
What a cutie
!!!!!!