Was this your post as well? https://www.backyardchickens.com/threads/duck-shaking-his-head-and-throwing-up.1543990/ I could have sworn that 3rd post had been by 'valeria-' instead of 'duckiedessert' when I saw it last night. Apologize If I'm mistaken.
But if so, I'd agree between the two posts that it would seem like a crop issue/blockage. You mention having him for a few weeks, and giving him treats over the last few days, does he have access to grit as well?
But if so, I'd agree between the two posts that it would seem like a crop issue/blockage. You mention having him for a few weeks, and giving him treats over the last few days, does he have access to grit as well?