As far as making yourself think you are pregnant, you can actually end up stressing yourself out so bad, that you may skip a period. And when your boyfriend says "he thinks he may have precum", he does. This is because precum is nutrient based, so that way when he does ejacualate, the sperm have nourishment, that is why it is called "precum", meaning "before cum". So, anytime, you are in a situation where your partner is going to cum, there will be precum without a doubt.
Be careful, and if you are that worried about it, maybe it is best you get on birthcontrol, or use a condom anytime you have intercourse, or even better, just don't do it at all. Worrying about being pregnant everytime you have sex, isn't fun at all.