Playing with Poo : How to stop?

Sorry for the rather disgusting topic but really looking for some advice on what we should be doing to combat this.

We have idential 8 year old twins - 'H' and 'F'. Both are none verbal autistic and both go to a specialist school.

'H' is potty trained in that he is able to go to the potty or toilet by himself although he does stil need nappies at bedtime at times.

'F' however is a different story. He goes to toilet to pee but he has seemingly no interest in going to poo on the toilet. This hasn't been too much of an issue. When he needs to poo he has been getting a nappy and handing it to us. we then sit him on the toilet and put the nappy on him so he can do his business.  Recently however he's reverted back to poo'ing in his bedroom or garden and unfortunately playing with it. By playing I mean rubbing it into the carpet or onto the wall etc.

When this happens we give him a good telling off and give him a shower while explaining it's not the thing to do and he knows better etc etc but we are struggling to think of a way to stop this.  It's only been happening for the last 2 weeks and we've decided to use this to be more 'hardline' in that when he hands us his nappy we intend to sit him on the toilet until he has done a number two but maybe this is the wrong thing to do?

We've always given him plasticine and foam to play with hoping this has the same sort of feeling/feedback but now we are wondering if it's something else? It's obviously disgusting and not healthy and his bedroom is not surviving this (removed carpets to put down lino etc)

In school it never comes up as he actually never goes for a number two in school. he saves it for home time!

Thank you!