@Azapuela he likes her enough to not want to deceive her. Simple as that. Using something like this is considered 'cheating' which is why MC gave him the choice to do it or not. Something achieved with a legendary pill rather than hard effort/strength that the girl wants is undermining both of their values and starts off the relationship with a lie he'd have to live with. ATM, he's just telling her he has a way but he won't do that because he'll try to earnestly earn her trust and love. If that fails then well, at least he tried with his own efforts.
These are martial artists and perspective into their ethics is important to consider when they make decisions like this.
And I get what you're saying, he could have not taken the pill and not said anything either. I think it's just for muh fall in love PLOT.
At the same time he's kind of showing off the power of his brother, showing her "it's possible like you denied bih" kinda thing.