Some people like to "act" in RPGs, some like to tell stories, and some want to roll dice. Most players I know expect all three activities, but in different amounts. It is unlikely you will ever be in a group where the balance is the same for each player and the DM.
In my opinion you have already done the right things by not pushing too hard, and having a chat to see if there is any problem. I think to follow that, you should accept the player for who they are, and make the best game you can with how they currently contribute.
Where you can, take a "yes" or a "no" from the player, and treat it as if the PC had said something a little more in character. Don't have the NPCs react to the lack of social niceties, but respond instead as if the PC controlled by this player had given the essence of what they said, but more appropriate to the situation. If you want, cover that with a little description (but avoid temptation to voice the request as the PC, narrate it instead).
If the player has a character with good social skills on the character sheet, and the system allows for it, let the player state intent without acting in character, but narrating desired outcome: "The fighter explains that he is a veteran of several wars, and worth more money than the merchant is offering. I want to persuade the merchant to offer more for the quest."
Continue to give them opportunities to contribute further, but don't make a big deal of it.
Best Answer
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.)