Local authority payments as income for Child Maintenance?

Hello, I’m hoping for some guidance on an issue I’m stuck on.
I’ve had a query from a client who is the non-resident parent for child maintenance purposes.  The original arrangement was set up in 1999 so would have been under the 1993 scheme, he was asked to pay £325pm.
In 2016 CMS closed the case as the child had turned 16 and the resident parent had provided no evidence / confirmation that the child was continuing in education.  18 months later CMS contact to say the resident parent has been in touch & child has been in education the whole time.  My client is now being asked to pay £273pm for the remaining 4 months until the child turns 20.  In the 18 months since the case was closed, my client has become a foster carer and receives fostering allowance from the local authority; employment situation has changed so he earns less, and has taken out 2 loans with repayments due to continue until next year, and cannot afford the amount set.  CMS have included his fostering allowance in their calculation of his income.  He has challenged this through MR but no change to decision - deadline to appeal 25th Aug.  Have trawled our info system and CPAG child support handbook (but only have 2016/17 edition) but cannot find answers to following questions:

. Should fostering allowance be disregarded as income?
. Can he make a request for the calculation to take account of his new loan repayments?  I’m not clear on whether the arrangement would now be taken over by the 2012 scheme, which seems to have pretty specific grounds for making a case for special expenses.
. If he challenges the decision, is there any risk of him then being asked to back-pay the previous 18 months whilst the child was in education?  If so would need to weigh up the risk.

Very grateful for any input!

Harriet