Have your vet check Mishka for a UTI or other infections. Many times this can cause a behavioral change that manifests in litterbox problems. One of my cats had dental problems & once we treated the infections in his mouth, he went back to using his litterbox like nothing had happened.
Oh - & clean any areas he has peed or defecated on with an enzyme product like Nature's Miracle. It will get rid of his scent entirely & help with him not returning to that spot.