[JoanneF]

Rather than a supplement, have you considered something in his food might not be agreeing with him?

 

[JoanneF]

You might be surprised at how many dogs don't tolerate chicken well.

It might be worth trying a diet without it. And, look for hidden chicken - here in the UK it's not unknown for a food labelled lamb, for example, to have chicken fat or by-products listed lower down in the ingredients.