It depends on a lot of things. When was the last time he ejaculated? Did he pee since then?
In general, pre-cum do not contain much live sperm. It comes from different glands than semen. It does however push out any live (and dead) sperm that is already in his penis. That is after all the purpose of pre-cum, to clean the penis from harmful things before he ejaculates his semen containing the live and healthy sperm.
When the pre-cum do contain live sperm, it seldom reaches fertility levels. It is normally a few thousand vs. the 40 million needed to impregnante you.
This does however not mean you can act irresponsible and take chances. There is always exceptions and you might just be the unlucky one.