At present, if he is paying voluntarily and the level is what it should be based on his last known wage and he is also contributing to the mortgage, then I would not approach the CSA. Contributing the the household via mortgage payments is also looked at CM in 'lieu'. To get both is a good result to be honest.
If he is getting cash in hand its VERY hard to prove the amount he recieves.
IF you do approach the CSA, and they assess him and it turns out the amount is lower than what you are receiving, it would be hard to backtrack..
What is the status of the separation?
Have you issued proceedings or the petition?
His only 'legal' obligation is CM at CSA level. Many argue that as he is no longer living in the
FMH then you should be paying him rent for the enjoyment of his half of hte house, however you would pay this direct the mortgage co.
But without knowing the full picture then its hard to comment.
If you post the following details then you will get more relevant advice.
Your respective ages;
The number of children you have and their ages;
How many nights the children spend with each parent;
The length of your marriage and any period of pre marriage cohabitation;
Your respective incomes;
Your respective outgoings;
Your assets - both soley held and joint;
Your liabilities.