PIP AR2 review - changed from ongoing award to fixed term

Hi, my 35 year old son who is on the spectrum but high functioning had the PIP AR2 review recently. As he has had some reliance on alcohol since the initial review 10 years ago we added that as a change for the worse. Also added further comments about his interactions with us reducing, increased self isolation and lack of interest in leaving the house (he has been diagnosed with Vit D deficiency and is on supplements). The result was that they have kept the award at the same level but now changed it from an ongoing award to a fixed award of 5 years and we don't understand why they would do that, as he has a lifelong condition that will not improve. We have an autism support worker who asked us to get a copy of the PA4 form but when we called DWP they said that the decision on his award was made internally and there is no PA4. They said they could transfer us to a decision maker who could explain their change to the length of the award but we have not done this yet. We don't think it is right to change it to being for a fixed term but are loathe to try a mandatory reconsideration in case it backfires on us. Does anyone have any advice for us on what we can do/say to DWP next? I am trying to get the autism support worker to join us on a call to a decision maker next week once I know her availability and we only have about another week to challenge. Thank you.