Thanks, @SirChenjins. He's fine in every other respect, as lively and energetic as ever. He would happily go for a run even in the heat (though obviously I wouldn't let him). He went swimming yesterday and had a great time. The vet checked him over quite thoroughly and couldn't find anything amiss.
He's been fine on plain chicken and rice, it's only when I try to add regular dog food that the vomiting occurs. I wonder if he could have developed an intolerance to something in the dog food? But maybe chicken isn't good for him either. I'll try some whitefish and see what happens.