He's blurring the boundaries because he's made his feelings really clear, and he's not confused. He can sleep over, he can text you every day, he can tell you that he wants you to be monogamous and stable and he doesn't want to play games.... you could even go on dates. Because he's told you that, for him, this is feeling a gap. You're being his not-quite-girlfriend whilst he doesn't really want one, and that suits him fine. He gets the sex, the affection, the attention, and you do, too.
The problem is that you're seeing him as a future partner, however much you don't want too. You're having to remind yourself of the guidelines and keep yourself and your feelings in check. They aren't the natural guidelines to you, they aren't the boundaries you'd have built or the rules you'd have made. So you're always going to be wavering over the control lines and struggling.
Be really careful, because most of the time, the guys in these things do eventually decide that they want a girlfriend - but it's not the person they've been with. He probably won't even feel bad about it. He might be sad that he'll need to give you up, but he'll see it that you both knew what this was. When someone turns his head that he does want to be with, that is worth the risk of being hurt, he'll be right there. All of us would be.
The age gap means you have less time to be dithering on this, and your feelings means it's bad for you - any connection you keep to him is going to be strained because it's shrouded in secret feelings and confusion for you and that angst isn't there for him.
The wise thing to do would be to end this because you're getting emotionally involved. I suspect he'd run for the hills but on the off chance he also has feelings, it would start the conversation between you to see if you could date instead.
If that is out of the question, try to build up to a point that it's not. Do things without him, don't text him much - dial it down over a few days so it doesn't look like you're upset with him, don't have too many sleepovers, treat it as sex. Keep yourself busy with other people. Remember that you'll be fighting yourself until you get rid of him, though.