Several INFJs aren't very good at determining their own feelings, as we use Fe, not Fi. He's just being cautious, trying desperately not to hurt you. So, the deal with the word "love"; for an INFJ, that word means a huge deal. We are highly romantic beings, and feel that "Love" refers to what's found in fairy tales. However, we aren't certain really what it feels like, so we're leary of the word, and want to be 100% certain we truly love the person before we say so. This doesn't mean he's having doubts about the relationship or that he truly does care for you, he's simply afraid of accidentally defiling the sacred word that is "love." Essentially, take nothing of it, and just give him time.
Also, for getting him to open, he has to do it on his own time. Assuming he truly does care for you, it will happen, and faster than for anyone else, but the Introvert will still take time. Just make sure to listen to him when he does decide to talk, be active in the conversation, but beware cutting him off. INFJs sometimes like to take breaks between sentences or idea to gather their thoughts. try not to take these breaks as him finishing his part of the conversation, and allow him to gather his thoughts for a moment. If you don't, you may scare him off from opening up for a time. make sure to listen and he will provide. Also, know that him not saying "I love you" is not a problem of openness, but one of his uncertainty, as described above.