Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first
Best guess on his income - and how many nights a month do they stay with you?
Then he is underpaying to an extreme extent
You should contact the Child Maintenance Service (CMS) - he should be paying 16% of his gross income a little less if he has other children in his home (when he does)
I hope that this is of assistance - please ask if you need further details
You are most welcome I hope all goes well