Sometimes, kids don't want to hear the whole story, just the answer to the question.
The reason his penis stands up in kissing scenes, is called "sexual arousal". That means he feels sexually aroused when he watches those, and it's completely normal and it's because he's healthy that his body is reacting that way. If you're religious, it's God's design. If you're not religious, it's perfectly natural, and nothing to fear. ;D
And then wait for more questions.
If you don't know where to begin, maybe you could go to a bookstore or look online and find a book or two on sexuality for pre-teens. I'm sure there are many to choose from these days, from different points of view (religious etc). That's what my mom did, she gave me a few books and a few days later asked if I had any questions about what I'd read. I was about that age.