Answer:
Yes, it definitely can. Pre-cum has less sperm than the semen released at ejaculation, but it still does contain sperm, and it only takes one for conception to occur!
You should always use contraception if you do not want pregnancy to occur. A little early thought can save an unwanted pregnancy. If the bare penis gets near the vagina there is always a chance sperm can enter, whether or not he has actually ejaculated. Pre-ejaculate contains sperm.