ESA Income related stopped due to overpayment (our fault not theirs)

Hi, first post so hope I do it ok :/
Our son who lives at home is on IR ESA support group & PIP enhanced on both, ESA has the SDP added as both myself and his dad both qualified too, his younger brother was at college and child benefit was paid. Now we have found out that since starting Uni last September meant the SDP shouldnt have stopped (I did call both his ESA and our Income support to tell them and they did say they would write to us with how it would effect us, but I lost track, my fault as hubby had to move from IS to Pension and I then had to claim UC which we struggled with, same time other son was needing help with UNI forms as we both have dislexia, not an excuse to miss this, I just didnt cope well)
ESA sent a text message to get in touch, they said we had to fill out a form and it would be sent out. IS10 form was sent with no covering letter, envelope or date to be back by. I am his appointee as he has both physical and mh issues, mh meaning he struggles with talking to people. We filled the form out and have sent it back. I called to see why his payment was late (June payment) and was told its all stopped until the outcome of the form has been done. I did say we had no idea it would be stopped immediately and shouldnt he of been told? Guy said 'sorry' and at least he knows now I agree we will need to repay the overpayment but surely they should let him know its stopped at the very least? His mh is in overtime as hes a bit ocd as well which doesnt help. I had hoped they would take it out of his ESA in instalments, has anyone ever had this please? Or know if this is right maybe? Any advice is welcome. Sorry its so messy.