I am an adult struggling to express my emotions and needs and fears in an appropriate manner. I think a child especially tween or teen would be developing their boundaries of polite/impolite... I fear the "peer group" influence because the bully humiliation tactics will not be useful life-enhancing skills for your son... as you pointed out, quite well, with your example of interacting with a boss.

My anger is from something else and "leaks out" at wrong people wrong times wrong way. Perhaps other stresses about the birthday party (will my friends be impressed enough??? will my party be cool enough???) caused your son's anger and his immaturity lashed out to be corrected and redirected by you. He should have said Thanks for helping me (but in real life kids take parents for granted until about 25 years old... then the phrase "wisdom of your elders" starts to make sense and by the way... actual adult brain structures are not "in" the brains of young people until about 25yo.

Sorry you had anger instead of "hey THANKS" today.