ESA wanting latest bank statement

Our son recently had a large SDP backpayment made. The DWP sent a letter out saying it would be disregarded indefinately for ESA. Then last Friday we had a letter asking us to confirm if his savings and income were below £5700. I contacted ESA on Friday to ask some questions as the letter was a bit vague. After 2 and a half hours I spoke to someone who unfortunately didn't speak or understand english very well and certainly wasn't understanding my questons. They then transferred me to the job centre and the poor lady couldn't access the information with regards to my enquiry. I have printed off his bank statements for our joint account where his PIP goes into, his statement where I am keeping his SDP which is an old savings acocunt in my name as he would just waste it and he asked me to hold onto it for him but he just asks for money and I send it to him, lastly his 2 accounts in his own name where his ESA goes into and his bills account. Has the SDP payment triggered something as he has never had anywhere near £5700 in his account until the backdated Sdp went in and it just said latest statement for his current account. So I have just sent the last months for each account, I also put a coving letter in and a copy of the disregard letter. Anyone shed any light on this or had similar. By the way he is on income related ESA with SDP.