Honestly, it could be either shyness or disinterest. I share the I and the N for sure, the other two can vary from test to test.
I am going to speak for myself, and allow you to see what applies.
* I generally only initiate conversations when I believe I have something New AND Interesting to say.
* I consider my life fairly boring because I do not have new or interesting experiences. Thus I have nothing to talk about
* I do not know how to talk just to talk. It is uncomfortable to try to force thinking about something to say.
* I am sometimes most at ease when we can just be quiet and exist in each other's company.
* Being sent things constantly with expectation to respond quickly gets draining very fast. But if I'm allowed to respond at my own pace, I appreciate the reaching out.
Social interaction has never come easy for me. But the bonds I forge with people that put up with that tend to be incredibly strong and durable. I can legitimately go years without speaking to someone and then interact with them as if it's only been a few weeks. And love them just the same. Not that I would ever want to go that long with an intended romantic partner.
Dealing with an introvert you are going to need to initiate conversation most of the time. Whether he is a strong introvert or not interested I could not tell you.
The best thing to do is to ask him directly. The more specific question the better. INTJ can feel awkward discussing emotions. Especially nebulous and abstract emotions. The more you can home questions into single response questions the better you will be able to get information from him.
Do not ask him "How do you feel about me?" That is a lot to analyze and try to think about how to say. Instead say "Are you looking for a long term romantic relationship or were you hoping for something more casual?"
Also, ask more about what he thinks and less about what he feels.