Talk to him
First step: do you really want to avoid sex during pregnancy
? If you aren't convinced he won't be either. Step two: a fair talk with him about your concerns, and try to reach agreement.
Most partners cooperate when they understand. If you need step three: i tell patients to blame it on me (the doctor.) but if you need step 3, think hard about what it means to you that step 2 didn't work.