He sounds like a very caring brother and stepson. His stepmother should count herself lucky.
Perhaps she sees too much of his father in him and it's painful for her being so close to her husband's death. Since you don't know what was really in her heart, and there's no proof one way or another besides suspicion, it would be better to assume that she wasn't cheating and had no intention of it.
So here we are with your son mourning the loss of his father and the only other people who were close to him before he died. He wants that closeness with them, to maintain contact and closeness with them, and to maintain some normalcy. But the thing is that his stepmother really is moving on and through her lack of communication, through her choice of when to get together, and her choice of terminology, she's setting some very clear boundaries and is actually communicating very clearly what she wants.
You can't force a relationship with someone who isn't interested, and that includes familial relationships. Since her children are minors that means that your son shouldn't overstep any boundaries or force the issue. I think he should absolutely remain present in their lives as much as she allows and his siblings want, but he may have to prepare himself for the possibility that he won't get the type of relationship he wants.
If he's ever able to sit down and have a face to face with her, I suggest he have a heartfelt discussion with her about this and to be very direct with her about what he wants and what she's comfortable with. Guessing and agonizing aren't doing anyone any good.