well, first: if you can't bring that type of conversation, even joking, with your partner... either you two started recently or there are some foundations problems (or many other options, but it's something too take a look at); second, you could probably try and avoid eye contact when he cums (hiding in the neck, focusing on other things). it's not ideal, but it can gain you time until you speak your mind to him.

(with all the good intentions!)