Refusal to award HRM based on SMI

My son will be 6 this month. He was diagnosed as ASD when he was 3 and with GDD a year later. He is completely non verbal, in nappies. Needs to be washed, dressed, fed. Constantly supervised. He smashes mugs, cups, glasses, ornaments, picture frames. Tries to climb out bedroom windows, undoes his seatbelt in the car etc. When we take him out we need to have him in a buggy for his own safety. He will not walk where we want him to/ hold our hand. He just runs and is obsessed with opening and closing gates so will get 'stuck' doing it and be unable to move on. He receives HRC and I've applied for him to receive HRM based on SMI rules as I have only just been made aware of them. He attends a SEN school and they have witnessed his extreme behaviour i.e. smashing his head off things when he is upset, refusal to go to certain areas/ rooms due to anxiety. The DWP took 2 days to look at the EHCP report, Paeditrian report, letter of support from school to tell us he wasn't awarded it because although he has delayed development they didn't see enough evidence of SMI. He cannot read, write, know not to put his hand in boiling water, or jump infront of a car. I'm at a loss as to what other evidence I can send them as I would have thought the EHCP was pretty conclusive, it even says significantly delayed communication and interaction skills, significantly delayed in his cognition and learning, significantly delayed emotionally. I think the fact they used delayed has given the DWP ammo to say he is delayed, but not impaired. Should I get an appointment with a paediatrician to say although he has GDD diagnosed he is not delayed but unlikely to catch up with peers. I feel it is the delayed word that is getting in the way. He has not made much progress in over 4 years in any of these areas

Dear Bren, personally I can only echo BIS's view. But I wanted just to send you my acknowledgement and admiration for the never endingly stressful job that parenting is for you, and one possible suggestion. We are supporters of Support Dogs charity, based in Sheffield. While there is a long waiting list, the empathy that a support dog can give a child with problems such as your son, can result in the child being much calmer. If you haven't already, this might be worth looking into.
Very best wishes, Denby