I think it's just cuz he didn't want to think about it, and he knew jaekyung would be super upset when he found out (since we all know he wants to give his ex an uppercut)
HELP JAEKYUNG????!!
NOOOOO WRONG SEME WKWKWKWKWKWKWKWK
HELPP OMG MY BAD IM SO TERRIBLE WITH NAMES
NO IM SORRY PLEASE
JAEHYUK*** I don't endorse that filth they just have similar names please don't shoot me
He's the one you're dating now so why keep anything from him? Even if the situation is a touchy subject clearly you still have very strong feelings about it and any partner would be insecure about the people that evokes such a strong emotion even if it is just hatred cause that means in a way you still care since it still bothers you so much. So talk to him about it tell him what happened and let him be angry with you so he can see why that anger is there and that he has nothing to worry about.