15 year old having sex

My kids father reached out to me to tell me he went through our oldest phone and found really disturbing things. He and his friends made a bet that he couldn't sleep with 50 different people in one summer and he accepted so he could be the freshmen with the highest body count. My kids dad found text messages of him messaging girls at his school and they were sexting. He was messaging guy and also seem to be having sex with people who LOOKED older. He doesn't know their age. His friends were saying he made it to 38 and had 2 weeks to get to 50. My kids dad asked what are we gonna do and I honestly don't know. I knew eventually my kids would start having sex but for him to be going out of his way to what appears to be having sex with anyone who says yes, I'm disturbed!

Edit: I have always trusted my kids. As long as they tell me where they are going at least my 15 and 13 year old I don't really pry. I never had a reason not to trust him until now.