I would probably side eye him, say "who exactly do you think you are talking to?", then order him to exit the room and return when he can ask politely. 

If he won't he can sit in time out. If he still won't I would pick his little butt up and escort him to his room myself. 

DH might have said the right things. But SS still ordered his father to come to his room, and his father did just that.

He's still got work to do on his reaction to the little dictator.

Me and that i answer to them.

i told hubby he'd lost his effin mind if he thought there would be a day i answered to his kids or they felt they could order me around. 
 

hubby finally found his balls late last yr and told eldest sd that she and her sister would never talk to me or treat me that way ever again that they were his kids(kidults) and i was his wife and they did not control me, lecture me or dictate what happens to me or my kids...

its been 9 months of them being quiet because dad day told them they weren't basically his miniwives and the miniwives title was redundant 

Just wow

On the occasions when my skids (5 and 7) bark orders, they are met with total inaction. We respond to "please could you do this," and we totally ignore rude commands. (That doesn't mean all requests are granted, just that we respond).  They've been told this enough that they figure it out quickly when they're ignored. If they do it to another adult, we do not ignore, and they are told to use their manners.  This is something kids figure out pretty quickly, in our experience.

FDH: "Did you just command me to do something? You will NOT bark orders at me!" (As he marches upstairs and tells SS to go to sleep)

Seems to me that your BF did exactly what SS wanted, no??

Your FDH needs to take charge here, his words don't match his actions, and SS5 knows it. No way a 5-year-old should have that much power in the home.

never to DH because he would have taken their heads where they stood.  However, Beaver lets the kids order her around.  

The skids used to come back from Beaver's all high and mighty and try ordering me around.

"Get Me Lunch"....NO, and who exactly do you think your ordering around...certainly not me. 

"Get me a toy".  Nope..and do not order me to do ANYTHING.  

It took a day or two for skids to fall back into line that you do not order me to do anything.

I remember when GWR and DD were about 14 years old and GWR ordered me to get him coffee. 

1.  Kids were not allowed to drink coffee at our house.

2.  Don't order me to do anything.

DD turned to him and said.

"Good luck with that coffee GWR.  Mom is NEVER going to give you coffee and she will never do anything that have ordered her to do.  Wrong house"

I had to chuckle over that one.  

Time for the boss to sit down his insubordinate employee and ream his little bossy butt out!   My girls went thru this.  We played the "who's the boss/kid" game. Every time they got bossy with me.  I say give the kid a sit down with dad and dad gives a life lesson to the kid just like he'd get in the real world. Age appropriate of course but if he wants to be like dear old dad then dad needs to teach him about his job and make it clear that you work you way up to that position, it's not handed over to a 5 yo.  Once that's cleared up in his sweet little head, I'd channel that energy into chores and encourage independence into launching skills so he leaves at 18.  

Does he hear his dad on work phone calls, bossing people around? Or is it just his perception of how his dad must be as a manager?

Dad may need to correct his son's perception of what it means to manage subordinates. That's it's not a dictatorship.