Poor baby
I wonder, since you see pieces of food, if the adult food is just so tasty that he's gulping it down without chewing it well enough. Of course it's going to come back up if it's not chewed. I have no idea what you can do, except for watch him when he's eating and make him slow down a bit.
If he keeps doing it, talk to the vet. You may want to call them anyway, they might have a good suggestion
