If mineral oil doesn't work for your cat (it didn't work for mine and I was never very comfortable giving it), try unflavoured, unsweetened Metamucil (available at pharmacies), a quarter of a teaspoon mixed in with his (wet) food every day.
The minute I see my cat is having problems, I start him on Metamucil for a few days and it always does the trick. The minute he's regular again, I stop. The more his own system is left to function 'normally', the better.
Agree with LR, wet food is best for kitties with bowel issues. Make sure he drinks plenty of water,