Looks like a black resin Casio strap. What I see would match a Casio F-91W, but there are other watches that differ from it only slightly that it also could be.
He has also been confirmed sometimes having worn a Casio Forester FT-600WB-5A, but that can't be the watch in the picture because its watchband is not all black.
(I've been nerding deeply into Casio watches recently after having ordered a
Sensor Watch. replacement PCB with programmable microcontroller.)