What can you do if a boy pre-cums inside you?

If you don't want to get pregnant and you didn't use protection, you can use the morning after pill. You can get it without prescription at the chemist's in some countries and states, otherwise consult your doctor. The morning after pill will work most efficiently if used directly afterwards. The chance of it working decreases with time, it should work in most cases until about three days after he ejaculated however.

If it's already too late to prevent pregnancy, consider abortion if it's legal in your country or state.