Not my child, but me. I was an 11 year old boy in 1994, and was exposed to a fair bit of porn. Initially in the form of a porn mag that a mate brought in to school, and then a couple of months later a friend found his Dad's video collection which we then shared round.
We weren't traumatised, we weren't horrified, mostly we thought it was funny.
The best bit of advice I can give is to explain to him that porn isn't sex. Sex is different, it's like the difference between a TV show and real life. Fewer explosions, more communication.
Porn is about the visuals, rather than what sex is actually about. Sex is about connection, making someone you care about feel good, not about who can make the other person scream louder. Porn can make sex seem violent, whereas in real life that's not the case. (Usually, but that's not a conversation that needs having at 11)
It's a difficult conversation to have, but an important one, and it's important that he not feel shamed for looking. His curiousity is normal, but he needs to understand that porn can warp his ideas of what sex is really like, especially when he's too young to have had any experience of the real thing.