If he takes a bottle of EBM no problem and will drink Karicare if he *has* too and you are only planning on him needing it if he needs to be away from you during a feed .... then I'd just keep offering the Karicare when you need to and not stress too much. If there's a time when he needs to drink and you're not there and you're out of EBM then at least you know he will drink the Karicare even if he's not thrilled about the idea
I never had to mix formula with BM (C would and still does eat anything going) but my girlfriends have done it. Make sure you make the formula to the instructions on the tin first and then you add the EBM afterwards. Also you want the formula and EBM to be the same temp when you mix them

Mind you, he still might not like the new taste and then you've wasted your EBM ....