Betta un-naturally small

I have a male betta whos very small, and compared to my other betta he is not as colored and has a hunched kinda back too. at first, i thought he was just young, but that does not seem to be the case. my other male betta is much larger and healthier. Maui, my red betta is quite healthy. But my blue-white fin betta Eric is really small and looks really unhealthy. any help would be appreciated. pls, let me know if it's something genetic, or if it's like a nutritional problem.

Thanks (also let me know if theres any specific diets, products etc i can use)