Sounds like he truly would like to stay in touch. In fact I laughed when I read it, because I have written almost those exact words to people I really like and do wish to remain close to. No door slam or even particularly bad feelings from the tone there.
I tend to worry when I get no response, even when I seem to have taken my time to write back. No news is always bad news. My feeling of getting a response back nearly two months later would be tremendous relief. I'm very bad at dealing with a complete absence of information. Even a negative response would be so much better than just wondering. You can at least work with that and there's some hope of restoring the friendship if there is at least some talking happening.
Whether it's writing to someone or cleaning, sometimes I'll put off doing it for a long while, but all the sudden wake up one day and have all of this energy and desire to write to one specific person or do go crazy with making the place look great.
It also really depends on my emotional energy at the time, especially if I want to be honest with the person about how I really am and show the proper concern for them. There was one person from this forum that I used to write to every day. I feel terribly bad for suddenly quitting, but I was working at a very demanding job, both time wise and emotions wise. I just didn't have it left in me at the end of the day. Even now that the job is quit, I want to write again, but now it seems like such an enormous task to catch us up (even though I don't think that expectation is there) that I have put it off.
Another person is someone that I corresponded with last year during a really hard time when I was living in an isolated community and someone I had dated for a long time broke up and then spent several months falling apart and acting very out of character. Our correspondance was a nice diversion and he was an interesting person to talk to even though I was not interested in him romantically. However, in the flurry of moving etc and then starting a new job, I suddenly dropped off. Even now, he responds right away when I write (he's on the other side of the world travelling), but it often takes me awhile to write back. I feel bad about this and can't even quite tell why I do it. Maybe he is associated on an unconscious level with that hard time in my life and so it conjures up some of those feelings again. I don't know. But again, I feel terribly guilty as he's an interesting, thoughtful person.
Lastly is one of my best friends from grad school. He ended up meeting up with his childhood sweetheart shortly after I went home and he had stayed on to finish his doctorate. Even though I like him and miss him, there was an undercurrent in our friendship where he would have been amenable to dating even though I wasn't interested. He didn't push the issue or even bring it up. However, after his fiancee was in the picture, I realized that I didn't want to be in too constant of touch because I didn't want her to feel threatened.
Anyway, I wouldn't say this behaviour is atypical, and in most cases it means that the INFJ likes you more rather than less. It isn't designed to be manipulative or keep you from moving on. Hope that's of some help...