Kidding about cheating???!?

So I have been with my boyfriend for 2 years. Things have been really good. But now he just seems to be taking everything as a joke. And for the last couple weeks he has been joking about cheating on me. I'm 5 1/2 months pregnant and he recently told his brother who is in the army about to get deployed in a few weeks & he surprisingly took it better than everyone guessed. At the place where his brigade stays at in Germany, they give out free condoms and as a "gag gift" his brother brought some home for him. I didn't find it very funny but I'm trying to be more patient and not let my hormones go crazy. LOL. But now that he has those condoms he says "oh I'm gonna need these the next few months" (meaning before my due date) or "I wonder if they'll enjoy it as much with one or without one" even though he says it in a sarcastic tone, it still makes me mad and I don't know what to do. He says he would never cheat on me and that me and his son are his whole world. But on top of me going through the pains of pregnancy this is tough for me to listen to.. help me ): ?