I might think showing up at his house with some beers and offering to watch the game with him might be a first step. I imagine he might not appreciate the intrusion onto his privacy but he also might really want someone to be there for him but is too proud to admit it.
Don't talk to him about it though. Don't even mention it. If you have to sit there for long bouts of silences, so be it. Just being there for him helps and when he's ready, he'll open up. Talk about yourself if you need too and some of the silly things going on with it.
Just my own experiences mind you, you know this ISTJ better than anyone, but from my own experience xxTJs tend to play this weird game when they're hurting where they want support and help but they want it entirely on their terms and you have to just play along to the scripted role they'll have you play. But thats just me.