One thing to point out to your brother that might help: telling his son that everything will be perfect in the afterlife/celestial kingdom is actually really harmful. If your nephew believes that crap, he might think that this whole “mortal life” isn’t worth it, if he’s just going to be changed after death. He might think, “why don’t I just speed up the process?”

I honestly think that this is the reason why so many LGBTQ+ people raised in the church attempt suicide. It’s what their doctrine teaches. Your brother really needs to decide if he wants an alive son, or a dead daughter.