change in ESA amount

Hi, My Husband has been in receipt of ESA for a number of years having been transferred over from Invalidity/Income Support, but wondered if anyone could throw any light on the latest ESA letter my Husband has received this morning from the DWP confirming and advising his ESA amounts back from December 2018, April 2020 and his present ongoing amount from April this year which is for £181.40 pw as my Husband claims for us both as a couple (I am disabled as well) part of this amount includes both a Disability income Guarantee payment and a Support Group payment for him. Only this Monday he received his renewal decision for his PIP claim which confirmed his continued and ongoing enhanced for both mobilty and care which we then found out had prompted a delay in his latest ESA payment being paid into our bank, but i was able to sort it out after being in a DWP call queue for over an hour! But this morning he has now received this letter advising that from 22 March next year his ESA will be reduced to £156.80 being a loss of £24.60 pw which is quite a substantial amount to lose every week. There is no explanation in this letter as to why this is going to happen but he will be 66 at the end of March next year, and I'm wondering if this is anything to do with it. If this is the case we have absolutely no idea where he stands as regards receiving a pension, up until he had to medically retire in 2003 because of his health problems he had worked consistently since 1972. I have been trying to get through to the DWP/ESA Department but just wondered if anyone had come across this before. I/We would appreciate any advice. Kind regards '