SDP Backpayment advice

Hi can anyone advice on what the rules are when you migrate from ESA support group to Universal Credit after a large payment for backdated SDP. Our son has been awarded over £22000 for backdated SDP and we are wondering if this will be affected when he migrates from ESA to Universal Credit. The gentleman who contacted me said a letter will be coming out that states it will be disregarded for the lifetime of the ESA award as it was an error on DWP side. But will this continue when he migrates and will it cause any issues. Will they want a load of bank statements as things have been a mess as for all I am his appointee his PIP payments always went into my account as he was on DLA as child then went over to pip when he was 16 so the bank details stayed the same and we just transfer his money over to him. But when he was 19 he applied for ESA without my knowledge and his ESA money has always gone into his own account which is something I have never had any control over. I have recently got a new account with joint names ready for when he transfers to universal credit. He has mental health problems, learning disability and memory issues so god knows what his account looks like. As he refused to make that one a joint account. I am ill incase anything on his sole account is out of order but he gets really angry just talking about it. I know he isn't over the limit of £6000 and we have also had to help support him finacially as he was struggling to pay his bills.