Messed up UC

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I am appointee for my adult son who has ASD with moderate learning difficulties. He claims LCW universal credit and PIP, has a part time job.
Last year he got some inheritance which I put into premium bonds for him £.5000. Then over time he saved more money taking it to £6000. Now I honestly thought £6000 was the savings limit…and only just found out recently that it is savings and what is in his bank account that needs to total £6000. A couple times he won some money which took him over so I put it in his normal bank account so that his savings stay at £6000, because I thought that was correct. I’ve clearly made a big mistake managing this for him. In the meetings we’ve been to at the job centre, they’ve never made it clear how it all works. All I knew was the £6000 savings rule. So as a result, his bank account had his wages and UC money in and he did not spend any for ages, so consequently over the amount for quite some time.
They did a telephone review today, and I explained that I misunderstood the rules and that he was over at various points. They said he could potentially be prosecuted and have asked for all bank statements from the beginning of his claim. Wish he never got the inheritance and I regret getting him a premium bonds account. Feel like I’ve let him down…he is totally unaware of all of this and he would get so anxious and worried hence why I am his appointee. I feel like I’ve let him down. Just wish it was all explained properly and this would not be the mess it is. They also questioned large payments coming out as he had to pay me back for stuff I’d paid for over time, like social groups he attends, driving lessons, etc. I am hoping as I’ve admitted the mistake it will just be recalculated and repayment, just worried me when they talked about prosecution.