Removing old urine stains from trousers?

Dad has three pairs of light coloured Khakis that have urine stains around the fly and zipper. I am going to try to purchase him a couple new pair of darker trousers next month. I do not hold out much hope, but has anyone had success removing old set in urine stains? The man Dad was would never leave the house wearing stained clothing. My step Mum, bless her soul, would not have allowed it either, but she is no longer living with Dad. (Long sad story, they miss each other terribly). I do not think Dad is fully incontinent, I think it is a matter of dribbling after he thinks he is finished.