DH had a court order to pay maintenance for his DS & DD until they were 16 or left full time education. The CSA was never involved.
At 15 DH started paying the money directly into their bank accounts rather than to his ex. He also explained to them that he would continue to pay the money while they were at school, college or university, but it would stop when they were working. The expectation was that they would eitehr be studying or working. Doing nothing was not an option!
Both left school after GCSE's and both started working - well, both already had part time casual jobs which they continued, initially.
Both are now working full time in other jobs.
He continues to support them with things like paying for driving lessons etc but no longer pays them the maintenance money.
In some ways I think this kind of unfair because if they lived with us, he would continue to support them living under our roof, even if they did contribute some of their wages to us, which we would expect, but it would be a nominal amount and probably wouldn;t cover everything. But it's also really quite difficult b/c where do you draw teh line, if not? As I said DH continues to support them with specific things which they would find difficult to afford, and will expect to do so until they are quite a bit older, I should imagine.
I think your DH needs to have a proper chat with his son about it.
DH was desperate for both his to go on to further education, and made it quite clear that he would support them, but neitehr of them chose that route.
Perhaps your DH could suggest to his DS that the CSA money will stop, but that he will contiinue to pay him an allowance directly (not necessarily the same amount) if he chooses to go on to futher ed?