Condoms are designed for the primary purpose of restricting sperm from passing into the vagina, as well as the transmission of std's. Most std's are smaller in size than individual sperm cells so you certainly do not have to worry about sperm getting through, assuming that you are wearing the condom correctly.
Only if the condom breaks. You should always inspect the condom after sex for tears. During rough sex, lubrication is especially important to prevent friction injuries and the condom tearing. Use a water or silicone based personal lubricant. He should also make sure every once in a while that the condom is still on his penis. Rough sex can make it slip off.