Help with PICA please ….

Hi my 5 year old grandson is non verbal and PICA , I am so worried about him . 
The only thing he eats food wise is plain cooked pasta , chocolate , marshmallows and crumpets with butter BUT will eat sand , play dough , dirt , plastic on wire leads ,  paint , paper, soft stress toys that have squishy liquid inside and recently grit for icy paths . 
When he went to the toilet ( still in nappies) the colour was red and full of grit . His little bottom was very sore as it would be but he never complains . 
I’m worried about the toxic elements plus what it’s doing to his little body he already has kidney problems. 
He goes to a specialist unit and they are aware of his eating but is there anyone else who knows of this problem or any advice ?