A quick update about his puking.
Found out it was a result from his food, not the Kong treat. Some of you might remember I mentioned he was itching a lot. Well I found some small lumps on his body, which seemed similar to what I get from allergic reactions. So I stopped feeding him his kibble (Arden Grange), and switched to a wet food called Nature Diet that has 100% natural ingredients.
He is no longer puking, and his itchiness has decreased. But he did gag up a small amount today from a bone I bought. So there must be something his body is rejecting.