Is it possible for an aunt and uncle to take custody away completely from his parents when they made mistakes in the past but are doing great now and have a house and jobs and love him to death ?

Loving a child to death does not make a good parent - it's about emotional and physical stability and safety. What "mistakes" in the past?

Yes, it's possible if the aunt and uncle can PROVE that the child is in unsafe conditions or exposed to unsafe conditions.

Depends on how serious the mistakes of the past were, and what and how serious the current mistakes are.