CSA demanding a payment way higher than partner’s income

My partner was in a relationship where two children were born. A girl and a boy, the relationship broke down and he walked away,, when they were quite small., leaving her with the house. She was supposed to pay him £15000’to buy him out of the mortgage but could only afford £7000.

He thought the best thing was to not have contact with the children as she developed a new relationship.

Sadly the son became very ill and my partner was told by a family friend that his son was in hospital, so he managed to get there just before his son sadly died.

The daughter now is 17, and the CSA have sent him a letter demanding £1600 a month saying he owes £23000 for last year. He is not earning anywhere near this figure.

They have been taking £80 a week from him which is acceptable, but changing it to £2000 for a first payment is now making us both ill with worry.

Who can we ask for advise and help.