Prozac as a treatment for isolation distress or separation anxiety is much more effective when coupled with desensitization training and possibly counter conditioning. If you’re not already practicing that it may help.
I am not an expert, but a 16 year old dog falls straight into the 'teaching an old dog new tricks'. I would totally drug up the dog to ensure it lives the (short) rest of its life peacefully and pleasantly. And I don't even like dogs.