It sounds to me like Sammy suffers from classic middle child syndrome. To me you are taking the right path but take it a step further and find something to do with him that JUST YOU TWO do. It helps give the boy identity.
also, I treat my kids like I treat my wife sometimes. I eliminate all their cause for complaints and if they still want to make excuses than I know there's something else to it.
Eliminate his cause for complaint and hopfully the problem goes away. If he IS getting attention and pulls a stunt like this again spank his but red!