If you're a high-risk driver, your best bet is to shop and compare as many companies as possible. As every company has their own risk assessment department, they will view your driving record differently.
If you're planning on selling the vehicle and have no intention of filing a claim for the damage, you don't need to tell your insurance company about the incident. You can just remove it from your policy.
You should be able to purchase a non-owners insurance policy and have an SR22 attached to that policy to meet the state requirement. It would be impossible to install an interlock device if you don't have a vehicle so I would recommend contacting your probation officer or attorney to discuss your options.
I would venture to say that most companies will insure drivers with DUIs, but unfortunately your eligibility will vary based on multiple factors like driving history and credit. Your best bet is to shop around with as many companies as possible to find the carrier that will insure you at the best rate.
Unfortunately there is no way to provide an accurate number or percentage due to the fact that there are so many variables when pricing insurance. One thing to keep in mind is that if he is already insured then the chance of his insurance company finding out about the violation is slim to none.