Such financial questions might be tricky for U.S.-based posters to discuss.
If he knows that you're filing, you should definitely tell him to postpone the purchase. It's a preventable headache for both of you.

Generally speaking, while it is true that the asset (car) and debt (liability) are shared between you, you can negotiate an agreement such that he gets both the asset and liability, while you get others in return so they balance out.
The split doesn't necessarily have to be 50-50.
In the U.S. the value of a car drops significantly as soon as it is driven off the dealer lot. He may be upside-down right from the start.