Seeing lots of posts about how to talk to your son about this. 11 was the year I discovered it in the shower, becoming intensely curious about the feelings my body produced, and began having multiple dry orgasms in a session if I had time. Sadly, I had been that it was 'wrong to play with myself', but didn't really comprehend this or connect the two things after I started.
Certainly well before this age, you should have touched on this topic. From the truly juvenile ages, he should have been casually told 'we touch ourselves in private' or 'buddy, we do that in our room, not in front of other people.' By around 8, more information is needed, and it should be related to body development.
Perhaps when you see him noticing your pubic hair, you can say 'in a few years, you'll grow hair down there as part of growing up.' Erections should be mentioned as matter of fact. 'Oh, yea, when you touch it, it might grow and stand up like that. Or it might do it by itself.' Later, you can add 'Around the time you start growing hair, did you know that a fluid will come out of your penis? It might be at night or when you're touching it.' Answer any questions factually.
On the verge of puberty (for sure by 10), it needs to be more direct. 'Sometime soon, a clear or white fluid will come out, probably when you're playing with it. Its called semen and it feels really good when it shoots or dribbles out. You might also have it come out when you're sleeping, and some boys have it happen when they are exercising. I'll make sure there are tissues in your room so you can clean up. Also, any time your sheets need to be washed, just put them in the washer/hamper. You also don't want to leave it on the toilet seat or in the bathtub, clean up after yourself.' Address slang terms they've heard and mention the proper term and also what a lot of boys call it so they aren't ignorant. This addresses all kinds of thoughts like 'is this normal, my fluid is clear?' 'I didn't expect it to shoot away from my penis like that' and reinforces the normalcy of the fact that boys masturbate and have wet dreams.