Have you thought about trying him on more fresh or wet food. The high levels of carbs in dry biscuits can cause some cats an issue. There are several threads on here about diet giving advice.

In most instances its a matter of trial and error, every cat is different so what will work for one doesn't always work for another, but if you have a look at the other threads they may give you additional ideas to try.

I hope you find an answer.

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