Well, with a scorpio sun and a leo moon, I think I may be able to shed some light on the issue, as I can relate to both sides. Both scorp and leo are of the fixed quality, meaning you are both very fixed and unmoving in your opinions. And, if you think you aren't think harder. The trick is to try and implement some of that leo humor into the mix. Scorpios love sarcasm, but you will have to be careful and pick the right time, especially when being sarcastic about them.
We are the sign of extremes (so true), so wait for him to be in a great mood, which can be hard to tell with a scorpio. But, here is the trick. Scorpios smile with their eyes. If we are in a good mood, our eyes tend to sparkle and have sort of a "bounce" to them. When you see that, start joking with him. Initially, don't say anything about the power issue, just crack the jokes and get him to laugh about anything. What you are trying to do is build trust, this is EXTREMELY important with a scorp. And, sorry to say, but we don't trust easily, so it will take quite a while, weeks to even months to build this. But, once it is there, you will probably start to see a change, he will probably start to let loose of the desire for power (if he is more evolved). He will still have it, but it won't be of a totalitarian mindset. We scorps, when treated fairly, tend to display great sportsmanship. But, then again, some of us are just plain evil.
On a side note, a quick way to build trust with a scorp is to not take the bait when we explode in a fit of emotional rage and explode yourself. Stay as calm as you can if he does this and allow him to take the time he needs to feel his feelings. He will probably leave for a while, but when he returns, welcome him back genuinely. Show him that you aren't like the rest of the world that shuns him when his emotions get the better of him. Let him know, THROUGH YOUR ACTIONS, that you want to work through it, whatever it is. If this happens enough, assuming he is at least somewhat evolved as a scorpio, you will have a friend for life. If you are the better front man, he will let you be it. But make sure he has an important role, and give him praise when he does something good. We scorps tend to be a little insecure, though we will never let you know it.
He just needs to know that he is valued within the band, that his opinion matters. But, if he continues to battle for total control, I am sorry to say it, but you will have to let him go. He will do nothing but drag the entire band down with a dictatorial mindset.