Cats like to mark their territory & by washing the bedding you actually removes traces of the scent they are trying to leave. You can buy pet pheremone spray from your vets which may help settle him & will not leave the smeall that his urine will. If you wash your pet bedding try to ensure that is in a low fragranced product. Some kittens like leaving there mark as a comfort sign - have you got an old t-shirt that you could perhaps wear & then place on the bedding? This will then leave your kitten with a scent that he recognises & will hopefully calm him. Failing that if the problem continues, get him checked over by the vet to make sure he hasn't got a urinary tract infection - hope this helps!!