I WONDER,IF YOU WERE TO BUT A NEW LITTER BOX,AND MADE A BIG FUSS OVER SUSHI,WHETHER HE WOULD USE,IT ,OR NOT.
I THINK,THAT HE IS A BIT YOUNG,TO HAVE HEALTH ISSUES,OVER HIS BOWEL MOVEMENTS,SO IT IS PROBABLY BEHAVORIAL.

AND PUT,THE NEW BIGGER BOX,AS HE IS A BIG CAT,IN THE PLACE,WHERE THE TWO BOXES WERE,BEFORE.
I HOPE,YOUR CAT,FINDS HIS WAY,AS THERE IS NOTHING WORSE,THAN CAT PEE.