I have another buddy who just started snowboarding, who is a little heavier but a good amount taller than me too. He's probably around 6'1 and weighs 170, you think he could ride a 154? I have an old, pretty stiff burton jussi that I've been trying to get rid of, and I'm sure he'd buy it from me if I told him it's big enough for him.
that might be too small for him since he's just starting out.. i think a 157 might be better for him to progress on
but it provides a lot of stability so if he's able to start out and progress on a bigger board he will be a lot better once he downsizes for more maneuverability
I don't think a bigger board will provide any more stability than a smaller board considering he's just starting out. IMO, the shorter, softer board will be more forgiving which may allow him to progress faster rather than getting frustrated and giving up on the sport with the bigger, less forgiving board.