What I do with the ashes depends on the personality of the cat and my relationship with them. There are some - Thisbe, Sesat, Simka, Bete, Artful Dodger, who I was so close to and/or were so home loving that their ashes in cedar boxes remain on one shelf of a bookcase. Others, who were so wild at heart or I didn't have a close bond with them for some reason have their ashes buried under a tree or flowering shrub. I couldn't scatter them, though I know many people do. So, what I'm really saying is, go with your instincts. Tigger was not your cat and didn't live with you. You did as you promised your dad and took care of him. I get the sense from your post that you would prefer to let his ashes go. Unless it means a lot to you to keep his ashes, let him go.