Read To Him
You should be doing this anyway because reading to kids is good for them in general, but it's really handy here. Both to encourage reading, and by mixing in stories of adventure you can let him use his imagination and foster that type of development.
Play Games With Him
You're already doing this. Keep it up! Make believe games are great, as they're pretty close to RPGs already. Board games with simple rules that kids can play are also good for learning how to follow rules.
The most important thing really is that if "games with parents" is an activity that he grew up with and likes, it'll be pretty easy when you're ready to invite him to an RPG by saying "we're going to try a different game tonight."
Play RPGs Yourself
Kids tend to internalize activities that are going on around them frequently as normal. So, if you want to introduce him to gaming later, you should try to game. If he sees people having fun doing that, he's going to ask what it is and be interested in what's going on.
You don't have to let him play right away when he asks, but it sets the stage for this being an activity that you do. (My son likes it when I roll dice, but he's too young to do it himself because he wants to eat them. One day I want to let him "help" by letting him roll for the monsters.)