My husband and I split while I was involved in another relationship. It took a bit of reassuring my b/f that he was not the *reason* my husband and I split. I won't say that he wasn't a contributing factor towards my feelings - it sounds much like what your S described as the realization that relationships can/should be different than what I had - but it wasn't because of him or for him that I left.
From my perspective, the best thing you can do at this point is continue to be supportive of him. Listen if he wants to talk, if he doesn't want to talk don't pressure him, and for the love of gods, don't badmouth his stb-ex even if he does. You're not being silly, but if you worry about it too much, you turn it into something about YOU and not about him - and I would say the biggest thing you can do to support him is don't make it about you.