Suggest Remedy To Overcome Internet Pornography Viewing

My son is 15 years old. My wife found his cell phone, which he forgot, in the bathroom. It had a picture of a vagina on it, and website addresses for porn sites. I don't know what to do. Do I tell him to stop? Do I take his phone away? I know that many kids nowadays look at porn. I looked at Playboy when I was this age. Porn is completely different. I checked his computer and found nothing on it about porn, just his phone. We babysit our younger nephews after school and it would be so horrible if they found his phone and saw this. Please help.