8yr old son inappropriate behaviour

[mamasaurusroar]

My 8yr old son is mildly aspergers and has ODD. He has always enjoyed being naked at home (which is not an issue), but he also grabs his younger brother's willy and will flash his willy at us. Every time we tell him it's not appropriate and if he wants to get his willy out to go to his room and do it there in private.
My husband collected him from school today and was told that he had flashed his bum at some girls in the playground.
Any one else have a son like this and what have they found helps to stop this sort of behaviour?