Income Support overpayment Disabled Student.

My son is disabled and started university in 2011. He started a couple of months late (November 2011) and Student Finance took ages to sort out his finance. I informed Income Support on the 1/10/11 of his change of circumstances and that I would send his Entitlement Letter as soon as Student Finance sent it. Student Finance sent the entitlement letter at the beginning of January 2012 which I immediately forwarded to Income Support. They now, out of the blue, sent me an overpayment letter demanding over £1,000. This is despite me sorting this out last January 2014. DWP informed me that there was no debt as it was a clerical error and I had kept them fully informed. My autistic son has finished university but is too scared to claim any sort of benefit even though he is entitled. They now say they will take a third out of his DLA which is his only source of income now.