What makes you think he is interested in a friendship? What if all along he only liked you romantically?
Also, the fact that he has a GF and that the conversations about feelings were not in person could explain why things are going badly I disagree with pink when she said ISTPs just "go for it" there's a lot that ISTPs don't say out loud that are tucked away deeply
The lack of communication in a long distance situation is pretty damn normal for an SP and I'm not sure about other SPs but I tend to be pretty limited in my txting as I just don't see the value of it over face to face... friendship might work over the phone but nothing more
Lets assume I'm right for a moment and he is only interested in you romantically, wouldn't his current behavior be expected? You're trying to have some kind of distanced friendship which may not be sitting well with him