I was gonna say..........
Don't fill his head with shit like she does, but maybe just try and point out the fucked up things she does like trying to manipulate everything to her advantage.
He will see what's really going on, and you will come out sweet in the end.
My son had not been getting along well with his mother since a few years after we had split up, around age 10.
When my son graduated, I was down in Alabama for his grad ceremony, and spent the rest of that night and the following day moving his shit out of my ex's place.