"The loss" I take it you are referring to the loss on disposal of asset, for the S Corp?

"Any recommendations on the personal use portion?"

The personal use is an annual event. It's a bit late to go back to 2015. There is supposed to be 100% business vehicle, because the corporation owns and operates it, maintains and insures it. The 15% personal use you describe is the taxable portion, which turns the amount into part of compensation (and is taxed through payroll) as taxable fringe benefit, and that expense is offset by "contra-expense" to the auto expense the corporation incurs.

And each year end, personal use of company vehicle is reported as part of payroll, meaning, W2, etc. Are you going to address that, or just dispose of the vehicle by "selling" it to the employee?

"the Scorp decided to let him keep the vehicle"

And it seems your person isn't going to pay for it? You don't get a free vehicle off of your employer. Are there other shareholders?