I agree with the Giro recommendation. They are a nice shape to fit a "full" face shape. I'd recommend looking at a leftover "Talon". I think it's a really good value in a helmet. My wife skis in that helmet, and loves it. Light, comfortable, nice features. Matches well with any goggle. She uses Smith and Shred. One of the nice features is that it doesn't seem to sit "high" off the top of your head.
There seems to be a lot of deals out there on the helmets. I just quickly searched and found this one, in a XXL:
Worth checking out, I think. Your son would approve of the Matte colored one, for sure!
In the used helmet world, the good old Briko Forerunner was a very popular helmet with guys who have BIG heads on the race circuit. I still see some, very lightly used, from time to time. The made them in a big size, and they are kind of long in shape….visually don't give the we are the mushroom look.
But…fact is that helmets are ALL about fit. It has to fit first and foremost.