Limited capacity for work and work related activities

I am looking for some advice of who I should contact regarding my son's universal crests claim, I am his appointee. He has been handing sick line in since May 2020. At the start of 2021 in January he got a letter regarding his medical which he was unable to attend as he had another appointment, I was Informed that another appointment time would be sent out. At the same time I contacted the dwp to inform them that he hadn't been sent out a medical questioner to fill in, three times I contacted them and he still wasn't sent one, he was then sent a letter regarding another work capability assessment for the march 2022 for which I phoned them up and said he would be unable to do as I was his appointee and I would be in New York on the date they provided I was told to contact the dwp and they accepted his reason for not being able to take part in the medical again I informed them that he had yet to receive a work capability questioner, more months past and at the start of July he had his medical without ever receiving a questioner, he was found to have limited capacity for work or work related activities due to severe epilepsy, he received a letter on his journal saying any back dated money issued to him would be issued in 7 days. He then received a notification on his journal to say his sick line needed up dated to which I wrote on his journal that as he had been found to have a limited capacity for work and related duties he wasn't required to hand any more sick lines in, I received a reply saying that he wasn't due any back dated payment and he was still required to hand sick lines in for another 3 months, I have written 4 notes on his journals asking for someone to contact me or even respond to my enquiry as to why he isn't due any back money despite handing sick lines in from April 2020 and why he had to hand more in for the next 3 months, I have tried phoning them and twice they have failed to contact me, who should I write to to regarding this thank you