I dont feel comfortable giving advice here, but you seem to be doing it anyway, you need to keep your son safe, whatever happens and whoever it is, your son needs to be safe. Kids dont ask to be involved in this disease but they're often the ones who suffer from it the most and have the least protection.
If any other person drove your son while drunk what would you do?
We often think because it's family we should keep it quiet or give them another chance, why should we? If someone puts our kids at risk it doesn't matter who it is!
Protect your son and yourself, your mom is grown up and will do whatever she wants, whatever you do.