A normal Plastic Plate Bowl rate to increase the height of the litter box would be about 5cm a day, but be very attentive to signs that your cat is not comfortable with the current height, and adjust the pace of raising the litter box accordingly.
Move the litter box to rest on the open toilet seat.
Keep it there until your cat seems comfortable with this arrangement.
Buy a metal bowl or tray that will fit snugly inside the toilet bowl.
The goal is teaching him to squat with all four paws on the toilet seat rim.
Start using less and less cat litter.