If condoms alone don't feel like enough protection for you, you could always double up on birth control. He wears a condom, you use a diaphragm or a sponge or something.
IMO, it makes sense to start the vasectomy discussion now. So many guys turn into reflexive pearl-clutchers when their wives mention the possibility! They'd rather that the woman just took care of things so that they wouldn't have to contemplate being the responsible party.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"