Don't try and force him, he's going to need time to become comfortable talking about his sexuality with someone like his parent. My mom knows better than to try and talk to me about my sexuality (she's tried a couple times and I won't discuss it with her because it's none of her damn business). This is one of those topics that should never be discussed without both parties agreeing to it or it being an urgent matter that must be discussed. Another example of that was the last time my mom tried talking to me about when I was molested as a child. I flat-out told her, "Don't ever try to talk to me about that again," and she still hasn't.
I'm 13 and still in the closet. He's brave to have come out. I try to avoid talking about homosexuality with my parents, and he's most likely trying to do the same. Don't worry about it too much.
I've known I'm gay for about 2 years, came out to my parents a year ago, it was however, brief, and every time they bring it up I sort of shrug it off hastily, So I suppose I'm the same way as your son in that fashion. I'm not sure why, but I think it's because I'm still getting used to the idea, and since it's not exactly the norm, it takes some time. My best advice is: Don't ask him about it a lot, just try to carefully approach the subject and see how he reacts, if it seems like he doesn't want to talk about it, change the subject and try it another time, or, if you can, just wait for him to bring it up. I'm sure you're full of questions and it must be quite difficult at times, but try to let him handle this at his own pace. I'm really happy to see that you're asking advice about this situation though, your concern for your son is admirable.
DON'T PUSH HIM!! I just came out to my stepmom last night, and it is actually extremely uncomfortable. I wouldn't be surprised if he is embarrassed a little. You've done the right thing of letting him know you support and love him. Now you just have to wait for him to talk to you. That's all you can do really. He's probably still getting used to it all.
Leave him be, let him work out how he feels by himself. It may not be big deal to you, but that doesn't mean it isn't a big deal to *him*. I would recommend you let him know that you are there if/when he wants to talk about it, and until then, just go on as normal. Don't bring up or avoid the subject. Maybe once he is 13 you could reccomend him this site if you are still worried. You are doing a great job, he just needs some time to think!
He knows that he has your support - this is important! As someone who doesn't have their parents support - trust me on this! Keep the communication lines open and in time he might open up. Remember he is a teenager - at that age I was secretive and didn't want my parents knowing anything I did. The things I would watch out for are depression, drugs, alcohol and sex - I know you don't want to think about it, but teenagers do and will engage in this behaviour. All you can do is educate and hope he makes the right choices.