Talk to him periodically giving more info as needed, especially if he asks questions or if things just come up. Keep discussions brief and simple, and they usually don't need to be very frequent. You can forewarn about dreams and teach other stuff later, even much later like in a year or so.

We give basic information periodically as they grow up. We probably have some sort of talk at least once a year. Sometimes the discussion comes up naturally in response to questions, and sometimes it's because we feel it's important for them to know something for their age level.

For example, a kid first starting kindergarten will be exposed to pornography, so we tell them what it is and what to do when kids are sharing that stuff. Even before then we teach that certain parts are private and other people shouldn't ask to see them and such. And through early elementary in response to questions and to teach why it's private we say something like "some day it will help you have children."
After asking questions, a child starting third grade last year learned a little about male and female anatomy and knows that's how children are conceived.
Last year a daughter in 5th grade learned a little more about female biology and periods and menstruation to forewarn her.
My high school children have learned all of the biology, and now they are reminded regularly about the sacredness of their bodies, the importance of families, etc. They're reminded they need to wait for the right time (when married), and why we have safeguards and rules in place.
Although we've been open and relaxed when talking to each of them over the years, their attitudes have varied. Some are comfortable and ask questions. E.g. a child in college asked a sex question this past year. But some are embarrassed and want the discussion over with. That's ok.
For all of them, we keep the discussion age appropriate. And we give out a little info periodically over time.

I'm talking like the expert, but the expert is actually my wife. She is very good at responding to questions and teaching information at an age appropriate level. I try to follow her lead.