Actually, on a funny note, I saw the title and I thought it was at first a discussion about pet-owners feeling 'Iffy" with old age and Health issues... I am 54 and feeling a bit long in the tooth some days, these days. I hope you were able to find some helpful answers, Because we do worry about our pets.
You will get more responses posting in the forum, this is posted as a blog post (I accidentally did a blog post once too, so don't feel bad if it was unintentional).
When you say he shakes, do you mean shakes like he's trying to get water off, or like a tremor?
My suspicion is that there is a neurological component to his problem, and the anxiety is a symptom and not the cause. These could be signs of epilepsy if I'm not mistaken, but there's probably a whole list and I'm really not the best person to be chiming in.
Try reposting this as a forum topic.