I got a pretty interesting question from @arcanemagic a while back about the rogue Scourge that Arthas lost control of during the Death Knight class mount quest and whether or not Bolvar is facing something similar during his reign as the Lich King and what exactly Bolvar got up to as a frozen popsicle all the time.
The rogue Scourge in question only freed themselves from Arthas not out of some sort of ritual of the Light like the Death Knights at Light's Hope but because they were consuming the leftover Life magic of a fallen red dragon. Because of this horrible combination of life and death, they were able to shake off whatever control the Lich King held over them. Now, granted, they weren't very intelligent, to begin with, so they just kept feeding on the life force of this dead dragon. It wasn't an issue of reach—Bolvar could still communicate with the Deathlord even that far out.
So basically, no, Bolvar's hold over the Scourge hasn't relented. If anything they are tightly contained to the ruins of Ice Crown. From what little we've seen of Ice Crown Citadel, it seems to have fallen into a state of ruin. Bolvar has no desire to make an undead war machine that will sweep up everyone in its path. He seems to have a very clear direction towards his anger, as well. He states that he won't command the dead to leave you alone in Ice Crown Citadel, but you have to remember you're also trespassing and capable of doing great harm. Either it's a case of can't or won't when it comes to curbing the Scourge's desire to kill those who are not Scourge.
I personally headcanon that the Lich King has all of the terrible traits of the individual wearing the helm amplified. Bolvar was held between life and death by the flames of the red dragons and was tortured by Arthas and then made Lich King as a result. While Bolvar in life was a very forgiving, calm, and patient individual, Lich King Bolvar has his grief and anger multiplied. The dragons, no matter the flight, have his ire. He has no issues with commanding the Deathlord to interrogate dragons at Wyrmrest Temple with force or send the Deathlord into the Ruby Sanctum where they can choose to destroy the flight's last brood.
In the future, I think this hatred of dragons will manifest again. But for now, all is eerily quiet from Ice Crown. Bolvar keeps watching over Legion activities through Amal'thazad and his agent in Acherus (and, to a lesser extent, what the Deathlord is capable of). It's safe to assume he has a firm grasp on what happens outside of Northrend.