He knows that if he says, "Yes, we're officially in a relationship", then it's quite possible that all the other "expectations" will open up like floodgates (i.e., "OK, now that we're in a relationship, I need you to do A, B, C, and D.") Some are small daily demands, others may be big picture, life-adjustment demands. I don't know if you personally would do that or not, but I know for a fact that there are people out there who do it. And I'm not falling for it. I want my freedom. I want a partner who allows me space and independence, not a mother who demands things from me. Therefore, I'm keeping you at a distance to preserve my introversion (and, in turn, my sanity). Once you've fallen for it once, you don't fall for it again. I'd rather err on the side of "caution" (tell you we're NOT in a relationship even if we functionally are, rather than say, "Oh yeah, we're definitely in a relationship!" when I'm not entirely sure I want that with you yet).
For an introvert, relationships can be extremely taxing, demanding, and consuming. If you aren't careful, the relationship will chew you up and spit you out and 3 years later you'll be thinking, "I'm not even myself anymore. How did it get to this? I'm not happy and I'm not doing the things that I enjoy and I love." So, it's sometimes a preservation thing. I'm going to proceed slowly and cautiously to make darn sure that I maintain my own sense of self and my own happiness. The absolute last thing I want from a relationship is to be changed or warped into someone else's mold, expectations, or demands. Let me give those things to you freely, of my own volition. Not because I said, "Yes, we're in a relationship" and so you then thought you had permission to have your way with me. It doesn't work like that. I'm still an independent individual. If you want me happy, you'll respect that. If you don't want me happy, well then go ahead and do your thing - place your silly demands on me. We'll probably crash and burn and you'll be left wondering why. Demands and expectations are basically "conditional love" - it's basically a way of saying, "I need you to behave like this - the way I want you to behave - or else we're going to have problems." Well, simply put, I don't want to behave according to your pre-set conditions. I want to be me. No more, no less. So, no, we're not in a relationship until I see that we're both mature enough to give each other space to be individuals.