I have opened myself up quite a bit. However, It seems like he's playing catch up with me much of the time. That's my concern. The increasing sense of vulnerability that has not been reciprocated. He has opened up in the last few months having become much more comfortable. He says it feels really good to be with me and he enjoys our time together because we have a lot of fun. But I'm not sure the feelings go any deeper. With INFP, they do, don't they? I don't know.
It's hard to say....often my feelings are much deeper than I express. They can be 1/100th of how I really feel. Other times, they are rather accurate expressions of a shallow feeling. What adds weight is the behavior that goes along with the words. It can be hard for an INFP to find adequate words to express feelings, so sometimes we show it through rather indirect means. It's too bad he did not take your expression as an opening; my experience with ENFJs (I have less with INFJs in this way) is that their gushy-ness helped me to open up & find ways to express positive feeling (to the point where I felt uneven sharing, with more on on my end). On the other hand, I take a lot of time to warm up in general, so subtleties mean a lot more from me than most. If he's already shown signs of becoming more comfortable with you, then there's a good chance he'll continue along those lines.
Sorry if you've already mentioned this; but have you directly asked him how he feels? I don't mean to
demand anything, but to ask him as he asked you. I agree you should be allowed to expect some reciprocation as far as expressing feelings go; I think it's okay to say, "So when are you going to tell me how you feel?". If you ask playfully, that may put him at ease to talk.
The very fact he was intent on knowing how you feel says a lot in itself also. If he didn't care much about you, then he wouldn't care about how you felt enough to ask so directly (as you're right, that is rather brave for an INFP).
He tests me ALL of the time. It's only after the conversation that I figure out what he was actually saying. I'm constantly reading between the lines. It feels a bit intimidating to constantly feel like you're being scrutinized. However, I figure either he likes me or he doesn't. There's not much I can do but be myself.
Of course you can't...If you're unsure about how he feels, consider his behavior. It says a lot for me to make time for someone, to see/call them regularly, to become comfortable around them & enjoy my time with them, etc. I can't speak for all INFPs, but I'm a rather solitary character who needs a lot of space, and I don't make a lot of time in my life for just anyone. I'm apt to disappear pretty quickly when dating someone once I decide they are not for me, and that's usually relatively fast. I realize you want/need confirmation in words also, but actions do say a lot.
If it's any comfort, I don't really "scrutinize" people when I toss a "test" out. It's almost unconscious to begin with, and I respond in a more emotional manner to it than an analytical one. If a person repeatedly failed, I'd be withdrawing, not slowly opening up, so it seems you're doing well (if he is similar to this INFP anyway).
but I believe I'll commence with a kiss. 
Kill them with kindness. That's what I always say.
with the comment. 
