my cat wont use the litter

My cat, oreo, is around 13-14 and he wont use the litter anymore, he always poos under my gaming desk so i put a litter box in my room, near my desk. he still wont go even if the box is clean. i moved it 3 different places and he still wont, and when im at my desk hes even pooed by my feet when i wasnt looking. any help/tips? thank you!