Maybe not GTO specifically, but I got the impression that he was playing the math and doing it well (especially since it came down to the two of us at the end).

The tell was a very common one. Anytime he liked what he saw, either hole cards or on the flop, he would quickly glance at his chip stack (thinking about a bet). Probably didn't even realize he was doing it. When it got to head to head, if he glanced at his chips...I'd fold early. If he didn't, I'd bet into him until he eventually folded.