My ex is suing his son for balance he feels is owed on a used vehicle he helped his son purchase. There was no agreement, written or oral. He is taking him to small claims court, the court date is tomorrow in Weld County Colorado. Without proof, something signed by my step-son, how can he sue him?

It is possible to sue someone on a verbal agreement. It is harder but it can be done.
The Judge often has to resort to who they feel is telling the truth and who is not.
Then they go by what the plaintiff can prove by their statement that sounds within reason.