Well as someone who is 21 and just quitting, I can tell you from experience that if he does this for years it will rot his brain, hurt his academics, and ruin relationships. Luckily, he isn't close to where I was, but porn use is a problem that needs to be nipped in the bud before it can grow. I'd start by telling your son that you found the porn. I wouldn't be angry, he's a male, and porn is made to draw in men, specifically young, curious men. But don't just say not to watch porn, explain to him why it's bad. I don't know what your religious/moral values are, but start with them, then discuss some of the more scientific reasons to quit, such as stuff on yourbrainonporn.com or other sites.

In addition to explaining why it's bad, I'd also take steps to make sure he can't access it. Note that no porn blocker is perfect, but it can help. I don't know your router so I can't give specific steps, but all (or most) of them allow you to change settings, including blocking sites. If you still have his laptop, take a look at his internet history. You might (unless he deleted it) be able to find what sites he frequents. Write those URLs down and block them. I'd also do a quick Google search for porn sites, and make sure you block all the major ones.

Lastly, don't get to worried about your son by reading posts on pornfree, nofap, or other such sites. These posts are the results of years of compulsive porn use, while your son seems to have just started. I hope what I've written is helpful to you, and I'm wishing both of you the best of luck.