yea that doesn't sound so much like only child syndrome as not discipline enough syndrome.
id just stay on him, its not gonna be easy if he's had it easy at home, mom and dad letting him run things, set those boundaries and give yourself and him two weeks for it to be rough for the both of you to adjust. that always helps me in my mindset knowing its going to be rough and giving myself grace. Id stay consistent in redirection again for two weeks, then reassess if needed, constantly explaining to him how to play with others, and in two weeks time you should see dramatic changes.
but you know this kid better than i do, id say make a battle plan for this next two weeks, give yourself grace and patience in this time and don't expect anything until the time is over and then reassess and you should be pleased with your progress, his progress
