As i tried to explain based on your other thread about the amount of income support you get you wouldn't end up with any more money overall even if you did take it to the CSA. The CSA are known for not being fabulous, so you're best not getting them involved if possible really, they've always been shit to my mum. But because the rest of your income comes from the government you will never be allowed to keep more than £20 per week of the maintenance jason gives you. So if his maintenance went up by £40 per month your IS would decrease £40 per month.
As you said earlier, only worth it in terms of getting one over on him, you won't benefit.
P.S- slight hint, perhaps stating the obvious, if you have benefit related questions why not ask your friend who works for them?!
ETA: Just looked on the government website based on him earning 24k a year and paying off student loan which he now will be this is what it said:
The weekly amount of child maintenance is approximately: £43
If you're claiming income-related benefits and you receive child maintenance, you may not receive the full amount of benefits from Jobcentre Plus. You are allowed to keep up to £20 per week of child maintenance before it affects your benefits. By law, if you claim benefits you must tell Jobcentre Plus about any child maintenance you receive