Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
While I do appreciate you are keen to try Vetoryl to see if your lad responds, I have to say that this is not a medication one can try in this manner. The reason is because using this drug in a dog that isn't Cushinoid (or using a dose higher then they need, which would be a risk without testing) could push your lad from making too much steroid hormone to too little. And not having adequate steroids in the body is actually more dangerous then having too much.
In fact, when we see dogs with too little steroid production, it is a condition called Addison's disease. These dogs (either by having the condition or by giving Vetoryl when they aren't actually Cushinoid) can develop a number of symptoms including weakness, dehydration, low blood pressure, depression, heart toxicity, vomiting, blood in feces, weight loss, trembling, shaking, and abdominal pain, collapse and death
. And just to give you an idea of how serious this is, when a dog develops these signs it is referred to as a "Addisonian Crisis" and often requires intensive emergency treatment.
Overall, while there are some medications that can be tried safely in animals where we have a high clinical suspicion of a disease; Vetoryl is not one of them. Its use in a dog that is not definitively Cushinoid could lead to serious harm or even death. So, this isn't something your vet in good conscious can risk with your wee one. And it something you truly do not want to try at home. The risks are much too dangerous, and if costs are an issue then it is better/safer for him to be untreated Cushings dog then risking this.
Please take care,