Migration from income related esa & housing benefit to UC affected by savings

I’m not sure if anyone can advise but my son who is 43 was diagnosed with schizophrenia in 2000 has lived in secured tenancy flat for 17years where his benefits and housing benefit was claimed with help from Richmond Trust. He had been stable for over 15 years until 2019 where he had a serious relapse and has only recently come out of a traumatic cycle of paranoia and suicidal thoughts which has lasted over 5 years.During this time his paperwork, bank statements everything was thrown out or hidden, he did not like us to search his flat so we were unaware of what money he had exactly. However I did rescue a bank statement from 2 years ago and noticed he had a lot of money in his current account, nearer £19,000, his outgoings and ingoings were pretty much even. When I asked him about it he told me that he has always saved some money from his benefit from first moving into his flat in case he was made homeless again and so went without heating in the winter and did not spend much on anything to the point that we bought him clothes and some day to day things. Now he has received a migration letter from his esa to uc and after talking with citizens advice she said he could not apply for uc and would lose both esa and housing benefit as it was overpayment. Sorry for the essay but you can understand my deep concerns of where we go from here. Obviously his paranoia stems from when being homeless in his teens which was very traumatic for him, he is much more stable now and being sensible, not drinking, eating well, doing walks and general much better wellbeing which is lovely but I feel this may tip him over the edge. Any advice as to how to proceed would be really appreciated.

Thank you for your reply David, this was how I felt was best way forward, he has not claimed PIP yet as his CPN has been unwell and is not aware of our sons savings so I have left a message for him to contact me when he returns to work. His migration notice has to be completed by 3 April but obviously he will not go this route at this point in time, should it be fairly straight forward for him to go on to uc once his savings are below the allowance? Also does PIP allow help with housing benefit as my other concern is my son has always had his rent and council tax taken at source so it goes straight to the Council and not in his account? I would be reluctant to let him have to pay this himself as it has been less stressful the other way. Thank you again very much for your help