There is a vast difference between being too impaired to drive and being "drunk" to such a level that all perceptions are entirely altered.
The defense can raise whatever it can to try and raise reasonable doubt in the prosecution's case. But, the witnesses and the driver would have to be pretty darn wasted for them to have all their stories disregarded by a jury on the grounds of their inebriation.
I do hope your son has obtained legal counsel.
- Carl