I often do this to a lot of my friends who I see almost everyday. Because ISTJ's are not as in tune with their feelings because it is auxilery, it could be likely that your friend has a difficult time processing his feelings as well as expressing them towards you. Therefore, causing him to go long periods of time with communicating with you.
As for speaking to you again as if there were no problem, ISTJ's have an incapability of apologising, even if they feel that they should do this, they have an they'll-get-over-it type attitude which is extremely common in ISTJ's.
The best thing to do would be to confront him about how you're feeling. Seeing your anxiety will solidify to him, how much he and your friendship means to you, and because ISTJ's are more likely to accept data once it has become concrete for them, he is more likely to take your feelings into consideration after.