Kids are SO SMART, and I would take advantage of that by explaining to him that sometimes people say things that they don't mean to get what they want. He didn't really mean that he wouldn't be his friend, but he really wanted that puzzle, so he said that just so he'd get his way. Explain to him that this is not the correct way to ask for something and use examples on what the proper way to ask for something is. Then, I'd tell him that he is so lucky because he can make choices for himself now that he's getting older. He could say "well, I am playing with it, but I will let you play with it when I'm finished", or he could say "I am playing with it, but you can help if you want to". I wouldn't get the teacher involved, unless you already have. Try to let your son develop his own social skills through experience, whether it be bad or good. These are lessons that will form his character, and the greatest part about them is that they're FREE!! So, help boost his esteem by telling him that it's his choice, give him options, and let him take control of it. He's got to learn somehow without intervention from the parents or teachers.