Hi harrison22 I used to work for the CSA and the way they worked out what should be paid was for 1 child it should be 15% of his pay after tax, NI and any pension paid has been deducted. �20 a week seems a very small amount but be careful if you are receiving benefits and you try to claim through an official agency eg CSA you used to only get �5 a week. Try this link to the CSA site and it will let you work out the amount he should pay you on line.
http://www.csa.gov.uk/new/calculate/
Good luck