my partners ex not doing rite by there son

my partners ex is a mother of four children the oldest boy is my partners an the three girls are someone eleses who has no left her he does not pay for the three girls and never has but my partner has always payed for his son. my partners son has told him that the money he gives his mother for him goes on the girls an he always feels left out,an does not want to live ther he is 12 years old an spend most of his time in his bedroom when he home what can we do she is his mother, but dont want to see him upset or feeling not loved and let down.