Boyfriend is good at sweet talk, but I don't like to hear how to tell him gently.

If words are not so crucial, why do you have to stop each other? What do sweet words remind you of? Maybe this is more important than how to refuse him. A simple guess: if you don't like it, you just don't talk about it. I think this kind of sweet talk will make you feel a little formal and insincere, right? Or sweet talk makes your relationship too close, and you feel unsafe? It is understandable that you don't like his sweet words. You are afraid that telling the truth will hurt him, which means that you have an impulse to hurt him subconsciously.

If you have to express it euphemistically, you can say that this kind of sweet talk itself can increase our feelings, but if you say too much, the efficacy will weaken.