Waterbaby, I'm sure you'd agree it's important to emphasise that the children in your last example were suffering from post-traumatic stress disorder. They were on board the ship when it sank. Their lives had been in imminent danger, some were bereaved in the disaster and some had witnessed the deaths and serious injuries of other people. They were severely traumatised, more so than if they'd just known someone who had died violently.
Whymummy, your son may be distressed, and may need to talk about it, just as Waterbaby has said, but please don't fear that he will become aggressive, antisocial or suicidal. It will help him that all his school mates are going through similar feelings, unlike most of the Herald children whose mates could have no idea what it was like for them.
From what you say the school seems to be handling it well. It's good that they got expert help straight away and are keeping you in touch with what they're doing and planning.
My daughter is nearly five and she is just trying to get to grips with the idea of death - she wants to talk about it a lot, wakes up in tears sometimes saying 'I don't want to die - I'll miss you'. And she hasn't known anyone who died, it's just that she's at an age to start understanding the idea but not fully understand it.
Will be thinking of you and your son,