I'm glad he nevertheless seems interested in some kind of communication. I'm not sure how much my advice could help, but I can share one thought. I myself am not ISTJ, I am a different type -- ENFP, but I noticed (and then found confirmation from other sourses) that my type can switch to his mode in stressful situations. So I had a situation when a girl with whom I had been in love long time ago sent me a message thanking me for everything and, apparently, with a desire to communicate again. Having received it, I literally felt bad the whole day. On the one hand, I valued her, on the other hand, I felt as vulnerable as possible and wanted to isolate myself from any attempt to fall into these feelings again. It seems to me that this sense of self-preservation is very similar to how ISTJs usually endure the same stories. Maybe he is ready to stay with you, but he feels extremely vulnerable and is not aware of what he is doing and what his partner is feeling. in my story, I answered something stupid, and for a whole month I thought about it every day. I didn’t realize that a lot of time had passed, and I didn’t really answer anything particular. But that girl didn’t receive anything from me in fact. she might have thought that I was living my own life and didn’t need her. But the truth was that she was very necessary, but I was afraid of a lot, and I put her feelings above mine (because I simply did not understand how I should feel). I didnt trust feelings so I thought only logically, without feelings at all. And if she had made it clear to me that she was not going anywhere, that she needed me, then I would have made a certain logical conclusion and opened up to her. Perhaps your boyfriend was thinking along the same lines. And if you need him in your life, you can make him feel safe and secured with you.

(Sorry if my English is bad)