When my SD lived with her mother...never. That was my husband's role. However, he also lived with me and was the parent who would pick her up on weekends, bring her to our house, and drop her off. My role was very intentionally marginalized.
I think that in your situation, things are different in that he did live with you for so long already and you are basically there acting as your husband's surrogate while he is out of state, so in his eyes, it makes sense for you to do everything that he would be doing if he were there. So yes, I think it's appropriate that if he's open to it, you continue to act as you would if your husband was there and you didn't have primary custody, which means weekend visitations, going to school events, helping out with activities if you can, etc.
It's an unusual situation where the regular roles of step-parents don't really fit. I know that if my husband were out of the picture for some reason and my SD went to live with her mom, I would be much more involved now than I was the first time she lived with her. Like your husband's ex, my SD's mother is not really equipped for motherhood and doesn't have the time or resources for activities, etc. so that's probably what I would end up doing.
At the end of the day, it's about giving the boy the best life he has regardless of where he lives. If you have the time and resources to do more and be more involved, then do so. Of course he may have decided to be with his mom (he's 9) but that doesn't mean that he should suffer more than he has to because your husband took on this opportunity. It's a lousy situation all around. Try to make the best of it.