It's not shota, I read the novel and it's the mc's complete rejection and repulsion of anything with him in his child form that encourages the ML to work harder at maintaining his true adult form, so he can win him over. The ML gradually recovers his true adult humanoid form for longer and longer periods of time, and meanwhile the mc's not interested at all, thinking about going home only, at the beginning he doesn't plan on getting attached at all (let alone to a beast stuck in kid form).
This is the novel:
https://www.novelupdates.com/series/jiang-chao-ge-and-the-spirit-weapon/
Pls don’t tell me there’s actually shotacon in this