I'm guessing enneagram 4w5, the struggle to stay his authentic self, to deal with the outer world while trying to stay autonomous, is very clear in his work.
Not sure about his MBTI, pretty sure he is INXP. I myself could not always relate to the main characters. I have read The Process, Metamorphosis, Amerika, most of his short stories and a part of The Castle, at some point I got bored by it. I liked Metamorphosis and the unfinished Amerika the most.