Try Benefiber, about 1/4 teaspoon, in a small amount of food. It mixes in
well, has no taste, and works well. It's just a bulk fiber that helps
things move through the digestive tract. Tasteless, odorless, and no
texture once it's mixed. Start out with a small amount daily for a couple
of days.
Once it's working, it can really push things through and give the cat too
much roughage, so don't give it every day. Recommended by my Vet.