I agree w/ the others in terms of switching foods asap.
This is going to sound super gross, but does the vomit "smell" like poo? If so, he may be eating his own poo, which can make them vomit. Switching to a better food can often help this -- but depending upon how long it's been going on, it may be a habit you need to break as well by using something like Solid Gold's S.E.P.
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