Absolutely normal, but he shouldn't have access to that sort of material until he's of legal age to view it.
Time to hide someone's porn better or add better child control to the internet.
I don't think a 12yo should be watching porn, however, a 12yo is naturally going to masturbate. It might be time to block access to these sites, i doubt he will bring this up with you as he'll most likely be too embarrased. All the best :)
It is normal for kids to be curious and explore but where did he get the porn from? How did he get the chance to be alone by himself for this long to watch a porn? Is this something recent? Or has it been going on for a while? I do think it is normal for children to explore but the content he is watching is not. Have you talked to him about it? What does he say? What did you tell him? How do you feel about the whole situation? I believe that both parents are responsible to talk to their children about these issues whether they are male or female although talking about it with the same sexed parent is probably easier. He needs to be talked to thoroughly. Since he is experimenting this far, you need to have a heart to heart with him. The porno's have to stop though. This is not healthy for a young child. I hope this helps you immensely.