Is Jojo still taking Vetoryl? I was a bit confused by your title about "untreated Cushing's" yet you mentioned he was on Vetoryl....
Have you stopped (or has your vet stopped) giving him this drug? And if so was it because of side effects or some other reason?
If he's still on it, has he had side effects of any kind?
Vetoryl (Trilostane) stops the cortisol production and can -if not monitored closely -cause serious side effects.
It is really important with Cushing's for the vet to monitor a dog regularly, take blood samples etc. And it's important that any side effect while on medication for Cushing's is reported to the vet quickly.
The medication is a balancing act to get the best benefit and stave off side effects as much as possible.
It could be that you don't want to risk the medication and side effects and your question is about what will happen if he isn't medicated at all?
What's happening with Cushing's is the adrenal glands are producing way too much cortisol which is a natural steroid. Over time cortisol can weaken the immune system leading to inability to fight off infections, and at risk of developing other diseases.
I haven't experience of Cushing's personally as I haven't had a dog with it. I don't know enough about it and have no idea what would happen to Jojo if he didn't receive any medication at all. There may be others here who have managed the disease without drugs. But I honestly don't know...
Can you ask your vet about how to manage the medication to minimise or eliminate side effects if possible?