It's so hard to let a beloved furbaby go, and sometimes people just don't want to admit that it's time. How is Sonny's quality of life? Perhaps you could discuss that with your mom to make her see if he is suffering.

I'm glad that he ate some tuna. Maybe, if he continues to eat, he will put on some weight?

Like kittycats_delight mentioned in her post above, I think getting an opinion from a vet with your mom in the room is a good idea. Perhaps they can help her make a decision regarding Sonny.
Be sure to cuddle him for us and please keep us posted.

**hugs**