I remember DS doing a similar "backwards" step. I'm afraid I put the game away and got it out again two years later, but it sounds as though that may not suit your situation! Brainstorming, some ideas, although I expect you've tried some or all of them:
- play in teams, e.g. you and him against DD7. You generally catch his errors, but from time to time, model an appropriate reaction to making an error
- let him play against a computer chess game (maybe one with an "undo"!)
- encourage him to play against himself, e.g. setting up and thinking about positions from a beginner's chess book
- offer to play him, but on the explicit understanding that the first time he makes any kind of fuss you (or DD) will go and do something else (and do it, without any remonstration or second chances)
- offer to play him with him being allowed to swap sides with you (or DD) at any time of his choosing
- offer to play him with him having the advantage, e.g. you (or DD) are one or a few major pieces down at the start of the game
- get him to practise good behaviour in competitive games he cares less about and then try to transfer it.
I don't personally like it when adults let children win (beyond the usual lack of concentration that can happen naturally) and I do personally insist on good behaviour at games as much as I possibly can, even if it means they don't get to play if they can't behave.
