Yup. As long as it's not affecting his behavior, he'll make a fine pet. It doesn't look like too bad of a deformity either. I once saw a corydoras catfish in a petstore who had such a bent spine he almost made a U shape, the poor thing.
Yianyian, the bigger the tank, the few the water changes you'll have to make. In his current-size tank, you'll have to change the water several times a week but in a larger tank, you'll have to change it maybe twice a week for a 9.4 liter/2.5g tank. I think your betta will thrive if you can possibly upgrade him as OFL suggests.
He has lovely coloring, almost looks blackish green.