Just thinking... hmm... how can you reward the behaviour you want him to repeat?

You could reward him for being brave... and keep trying to ignore the behaviour if you honestly belive he is putting it on. Perhaps even explain to him that you know he isn't scared at his dad's place, and (again, assuming he IS putting it on) that you won't tolerate game playing?

Sorry, I don't have any experience, I just didn't want your thread to go unanswered .

Nice to meet you today too, I hope the nappies didn't overwhealm you.