I guess the question is whether you want to punish your son and are looking for evidence of his suspected misbehavior, or if you want to protect him from some of the content on the internet in the future. If the latter, the solution is no computer in his bedroom, set up a family computer desk where anyone can walk behind the user at anytime, and make it clear that if he is found to be visiting sites which you disapprove, not only will his computer privileges be curtailed, but he could face banishment from online access. If he has his own smart phone, then you have other problems, but another measure on the Mac is to setup Parental Controls.
I know you were seeking technical advice, but I'm just trying to give you the benefit of my parenting experience. You can obviously take it or leave it. I have two boys, and this strategy worked quite well. Once when I did a little probing, one son said, "I'm not even tempted to look at X-rated sites. I don't want to lose my computer privileges!"
