@StuntNun I think you need to talk to his teachers about what grade they will think he will achieve. I don't know if exams are cancelled in Northern Ireland?
If he has a realistic change of achieving the grades set by Queens, he should be encouraged to put that as his first choice irrespective of an online open day.
Of course it is important to go to where you are happiest provided the Unis are equal but as you know, they are not.
However I feel you need to do more digging around why he would no longer place Queens as no 1. If ABB is aspirational then he may not want the pressure. In an already uncertain year he may want the certainty of a safer option.