You've already gotten a lot of good feedback, but something else occurred to me, just to consider.
In order to describe all the details and lengthy descriptions that your son has heard, it must take some time.
So my question is: where is this taking place? Is the other boy cornering your son on the playground, before or after school? Is it taking place in the cafeteria? Are there unsupervised times when the boys are alone, like some kind of free period or during bathroom breaks?
You might ask your son where these conversations are taking place. That would be something the school could address. Maybe your son is being bullied into listening to this filth and can't get away. The school should be a safe place where your son does not have to listen to this.
Wherever that other little boy is getting his info from, whether he's being abused or repeating something he's seen or being told, or no matter how bad his home situation is, it doesn't belong in the school. If there is a lapse in school supervision where one 3rd grader can tell another in detail about pornography, that needs to be addressed.
If on the other hand, your son is willingly following this boy around, say, after school or sitting with him on the bus, and allowing these conversations to take place, then I would say that it's important to help your son understand how to avoid this child, and intercede if necessary (pick him up immediately after class, or change his bus, or whatever).