Does he eat wet food? Or is there some he is allowed to have?

Try mixing in a bit of Metamucil. Not sure how much to recommend...kind of like parmesan cheese on tomato sauce.

We used it at the kennel where I worked for older cats who stayed 'bound up' for two days. (Two was normal for younger cats after they had just arrived - but older cats can tox up faster).

They never noticed the flavour of it, and it got stuff moving!

I think that'll work.