I've got a late primary age DS. You're already a lot of the way there just being aware of the issues and potential problems! 
Toys - there aren't "boys' toys" and "girls' toys", just fun, interesting toys and dull ones. (Get him thinking about plastic tat toys and TV tie-ins, versus stuff you actually get a lot of mileage out of playing with.
Challenging stereotypes: Google is your friend. If he comes home from school saying "girls can't do X", a quick google will throw up a wealth of women doing just that! (I think with us it was "girls can't be generals..." - you'd be surprised how many there are in how varied a set of countries!)
TV, especially ads: how are they different for girls and boys? Is it fair? Why are they pushing some messages and not others?
Sports: Unlike a PP, I let my son play rugby and football (he loves both). If your son wants to go in that direction, my advice would be "pick your teams carefully". Find coaches that stress fair play, teamwork and inclusion and bringing everyone up to a decent standard, rather than favouring prima-donnas and strategic diving in the penalty box!
I'm now entering what I consider to be the scariest stage - pre-teens and teens. I know that at some point, no matter how much I supervise his internet use and keep tabs on the parental controls, he is going to see porn etc. on friends' smartphones. So I have to prepare him. Some of this has been an ongoing task from really early. Teach about compassion and mutual enjoyment and consent in a non-sexual context right from the earliest years, and the groundwork is there for later.
So: "It's only a game if everyone's smiling...", "Your friend doesn't want to be bear-hugged/play-wrestled/whatever, right now...", "You don't have to kiss a grown up if you don't want to...", "You can say 'no' to anyone touching you if you're not comfortable...," "You have to listen to other people saying 'no' and stop, immediately..."
Now he's done the facts of life (from me first, then in school) I'm stressing that sex is fun with someone you like and respect, something you do because you both want to, that 'no' means 'no'. That people will show you pictures/movies of people having sex, and sometimes this will be mixed with horrible nasty stuff, because some people are horrible. That nice sex is not like that, and that if anyone shows him stuff that is upsetting, he can come and talk to me about it - that I won't be shocked because someone else has shown him stuff, that I'm here to talk it through and help him to learn a healthy attitude to sex.