Mum says my dad is bad #PAS

What can you do when your Ex is filling your child's head with constant lies. She has more contact with him than I do so has more opportunity to influence him. Luckily in my case its not working. I still have a great relationship with my son even though he is far away and contact is limited. But what can we do to combat this ? I have often hear her saying your dad is bad. He does this he does that. Its his fault we live in this place and have no money.