Beware, one of my longest posts ever!
His rage mode is his subconscious power atm, right? So in order for him to gain control, he would have to train in Rage mode without losing control. He probably already tried that but would eventually lose control because he had nothing to stop him from losing control and had to stop training. In order for him to attain control, he would have to practice while tapping into his rage power for set amount of times without losing control. Eventually after training for longer and longer set amounts of time while not losing control, he will gradually gain control of that power. The only problem is, he needs someone or something to keep him from losing control just in case he loses it in one of his training sessions. Is that the basics of at needs to happen?
The reason I say it's like Naruto, is that when he was younger, he had the same problem. When his friends were in danger or he was in grave danger and was powerless to do anything, he would tap into the massive reserves of power he had. He basically had the strongest evil tailed beast sealed inside him which had almost unlimited amounts of energy. When Naruto wasn't able to do anything with his own power and his friends were hurt he would become enraged and lose control of himself as he was forced to use the evil beast's energy. There were multiple incidents where he just lost it as he used the beast's evil power, even attacking his own teammates. One great example of him being consumed by rage is the Naruto vs Pain fight when his (Spoiler: future wife) was almost killed in front of him. Epic and probably one of the best fights in the series.
Throughout the series, one of his masters was there to help him gain control of the beast's power and tap into it without losing control because whenever he would use the beast's power, he would go out of control. His master would undo the seal while Naruto tried to use the evil power without losing control. And if he did lose control, his master would close the seal and stop him from using that power. Eventually he befriends the beast and is able to control the power through training and stuff like that. There's a lot more to it, but I don't want to explain the whole Naruto series right here.
How this relates to Dask you might ask. Well it's almost the same thing except Dask doesn't have a beast inside of him. He does have an unlimited supply of energy. And when he rages, he subconsciously uses his stored energy, and like Naruto, he loses control.