You could say "I'll be out of town" especially if you don't plan to be texting him while you are off for the weekend. And if you DO plan on texting him, and you think he might feel even more uncomfortable thinking about how you were talking to him while off with MrB, I'd probably want to say it for sure.
I have an ex who gets uncomfortable hearing about my boyfriend so I while I wont lie, if he asks about my plans I just say "I have plans out" or "I won't be back on the computer to talk until Monday" with the understanding if he ask more details he is going to hear that I have a date. I guess I think its good the onus is on him to not ask if he doesn't want to know, and not on me to have to be dishonest. Would you find yourself in a pickle if it comes out later you weren't totally honest with C?
Not sure if you're going to be comfortable with a long term DADT about mentioning dates ahead of time, (I can't remember how long you've been seeing everybody!) but I hope C is working on finding a way to be OK hearing about things. Have an awesome weekend too