Hi Ylva, Losing a real cat is so tough. I've had cats for 16 years now and I've lost a few too. It's never easy. I think an SL cat is a great idea to help your son get over the worst of the loss.
However, I'd like to make a suggestion. Since, the particular combination of KittyCat traits might not come along soon, how about considering involving your son in breeding the cat he wants. I don't mean that the 2 of you should go crazy with it like so many of us do (

). But you might be able to find a flame cat and gradually breed it with others to get the cat he wants. It might be fun for both of you.
This could be biting off more than you want, however, so I understand that you might not interested. Your post has motivated me to try and see if it's possible. If I can, by some miracle, breed the cat your son wants, it's yours. I'll let you know if I have any luck.