If you think that swallowing sperm can cause pregnancy, it is absolutely and utterly impossible.

The only way you can become pregnant is if a healthy sperm meets a healthy egg in the fallopian tubes. For this to happen it needs to be around the few days of your ovulation as the egg dies pretty quickly if not fertilized. If you can figure out how the sperm can get from his penis to your egg using oral sex, it is probably possible. You also need to know that saliva is hostile to sperm.

Even if you can figure out such a route, the chances are very very very slim. For example, you give him oral and he ejaculates in your mouth. You kiss him and transfer some sperm, he gives you oral and the sperm gets into your vagina - very unlikely that enough sperm will survive that ordeal.