First off, congratulations on starting!
I have a 5 year old niece who I'm extremely close too. My child is really going to be more of a sibling to her, than a cousin. Her parents told her my news the day the agency gave me the OK to start my dossier. I thought, given her age, it was waaay too early but she's been cool with it. She talks about it a lot (the three of them are coming to China with me) and is very excited but good about knowing it's not happening until next year.
I guess maybe it depends on how your son reacts to waiting for other things. Some kids can wait, other kids ask each day if this is the day. Which would probably drive you crazy.
The strange thing I've noticed about my niece is that she has absolutely zero interesting in talking about adoption. My SW kept giving me lists of books about adoption for her to "prepare her" but my niece is just very matter of fact about the whole thing, and not curious at all about why I don't have a baby in my tummy. Anyone else experience this?