CSA case closed 5 years ago.. Now chasing arrears?

Hi all, after a bit of advice.
My OH has 2 older boys via 2 previous relationships. 1 is 24 in Dec & the other is 21 in Oct both working full time.
To cut a very long story short OH has been paying via DOE consistently for the last 14 years inc calculated arrears payments. We notified CSA of the births of both of our daughters in 2002 & 2005 & this was considered. Case for 1st DS was closed in 2009 (for which we have a letter) due to him starting full time employment & no other communication since.
Case for 2nd DS was closed in 2013 with final arrears calculated & final payment being taken in april 2014. (again letter received with breakdown of monies owed.
OH has received an email from his payroll dept stating final payment has taken affect & DOE is now been canceled.

2 weeks ago OH receives a phone call on his mobile stating that there are arrears owing for eldest DS of approx £2000. OH stated we had a letter stating case closed 5 years ago no monies owing & also 2nd DS closed in 2013 final payment taken 2014.

They have stated that they have recalculated & it appears there is still an outstanding amount for eldest son? He said there must be some mistake & would like to see proof of all payments for both DS since CSA involvement via post & put the phone down. To date we have received nothing in the post & they keep calling on his mobile which he now refuses to answer.

Can anyone advise if they can do this? Can they start up the DOE again without his permission? Does any of this change with the new CMS being put in place?

any advice would be greatly appreciated.