K.D.
M.,
I also have a son about your son's age, the second child as well. He, too, has started yelling and bossing a lot lately. After reading everyone else's post, I think maybe it's a phase? I think all of us with more than one child could probably spend a little more one-on-one time with each of them to aid in combatting negative behavior. However, my son is bossy even if we spend an entire day alone! Every time he uses that rude tone I let him know it's rude and that I won't respond to whatever demand he is making until he uses his manners. I try not to step in when my children argue (I have three), but when people forget manners I do remind them. Unfortunately, these reminders haven't seemed to stop my son from being rude, at least initially. I just plan to continue reinforcing positive behavior and making sure he uses his manners (even if I have to remind him) before I comply =)
Good luck!