SDP Backpayment advice

So after reaching out on here recently I have an update and could do with some further advice. Our son has had his SDP added onto his ESA. I spoke to a gent last week and a payment went in on Monday the 14th April for 2 weeks backdated and then his new adjusted payment went in today. Then the letter came about his new rates and what had been back dated. No mention of it being backdated to 2018 when he became entitled to it. So the background is that he was getting full pip care and mobility when he moved out and ESA support group. We contacted DWP which they said they have a record of in March 2018 that he had moved address and that he was living alone. Nobody was getting carers for him and never has and he has continued to get full PIP and ESA Support Group. So after a phone call today I have been told he isn't entiltled to it back to 2018 but only from the 18th March when the is10 was filled in over the phone. I explained that we did inform them back in 2018 that he had moved out and I was his appointee but nobody was getting carers allowance for him and never has. She said she would put the information forward to the benfits office and someone would call me back in the next week. So am I correct that he should be entiltled to have it backdated as we did contact them back in 2018 to inform them of a change of circumstances. Any advice on where we go from here if they refuse to backdate it, is their anything we can do if we need to take it further as my understanding is that he met the criteria and we informed them of the change but nobody acted on it, so is the error on the DWP side. Many thanks.