I am sorry your brother lost his beloved dog this way.Scary indeed. My brother lost his well-loved, aging Rott X several years ago to it.They were playing vigorously with him right after eating, and his stomach flipped. These big breeds are kinda prone to it. I hope you enjoy many happy years with your new addition, he is a cutie! ❤
For big/heavy breeds one way to prevent the gastric torsion would be to feed them several times a day, but only small amounts each time. And of course no playing right after the meal, but that is often easier said than done.