There are NO cues to use on your horse because he has not been taught to respond correctly to any leg ques. He has been ridden ONLY with rein aids; and as you have found that the direction the reins take his head has absolutely nothing to do with where the rest of him goes.
Horses do not have to follow their noses. They HAVE to be taught to follow their noses. Horses follow their shoulders and where their front legs / shoulders take them. You have to establish shoulder, rib and hip control and not just point his head and HOPE that is where he goes. That does not work very well -- as you have found out.
Since he has learned to 'tune your legs out', you are going to have to establish a LOT of respect from him for your legs now.
You can start with ground work, but ultimately, he is going to have to respect the legs he has been tuning out. If I have basic ground manners in a horse -- meaning he will stop and stand still when I say "Whoa!", will back up lightly, move his shoulders and hips over, etc -- which takes me about 10 or 15 minutes on about any horse. The more effectively a person rides, the less they have to rely on ground work. Many professional trainers do very little ground work and save that for their beginners. If they have good safe basic ground manners and respect, they rest is all done on the horse;s back. After 4 or 5 rides, the horse is ready to ride out and go do a job.
I used to start more than 50 colts a year -- many that came not even halter broke or touched and many that were 4, 5, or 6 years old.
Their owners wanted to came back and get a horse that was ready to do a job and ride all day on the ranch. Two and 3 year olds just did not fit that job description, so they brought them when they were old enough to go to work the day they picked them up to take them home. These horses had to be gentled and had a lot more time spent with them on the ground than a gentle barn raised horse that came years later when horses were barn raised instead or range raised. Most trainers just do not want to spend that much time on the ground and want to get everything done horseback -- and DO get everything done horseback.
I have a logical order of steps I go through with a green horse.
1) Establish forward impulsion with little or no direction the first ride. Often times, if the colt is free moving, I lope them out the first ride.
2) Start basic guiding -- getting the horse to follow its nose.
I will start using inside rein and inside leg (bumping) if the colt drops a shoulder and comes off the round-pen fence and will bump with an outside leg to get the colt to cut straight across a round pen to either change directions or continue on the original circle. I bump the inside rein and bump with the outside leg but not press steady or pull steady and not make any contact if the horse does the right thing.
3) If the horse is not a 'scooter' (tucks its tail and tries to get away from the rider by scooting forward) I start in the round-pen for a few minutes and then walk and jog outside all around the barn yard, around corrals and parked trailers, tractors, etc. They get used -- right away -- to going forward no matter what is around. I cross ditches and dead-fall trees (that we purposely have not cut up and removed) and may go to the pasture. [I will ride one this green only a pasture that does not have loose horses in it.]
4) Next ride will be 5 minutes in the round-pen, 15 or 20 minutes around the yard or pasture, and 15 or 20 minutes in the arena.
This is where I will start leg yielding exercises. I use the far fence (away from the gate) and go down the fence making the horse stay straight. [This may take tapping on that shoulder with a bat if the horse has already learned to drop a shoulder and 'lean' on that outside rein.]
Then I go all the way down the far rail and do a half circle to the inside and leg yield back to the rail. [I start out with a 20 to 30 foot half circle.] The first few times it won't look much like a leg yield, but within 15 or 20 minutes, the horse will noticeably 'fade' away from the bumping inside leg and will let me take more hold on the inside rein without turning to the inside.
It is like a dance. It is a constant give and take, a little push and little pull, a little steadying, a little dominant on the inside leg, a little more dominant on the inside rein, a little more 'steadying' with the outside rein. It is all timing and feel of when and how much where. You never take the horse's head past the center line to the outside because that will instantly put all of the horse's energy into its inside shoulder and will cause a horse to drop that shoulder. They learn this very quickly.
Once a horse has learned to respond properly to the lateral aids (dominant aid is the rein and leg on the same side), they are ready to start teaching the proper response to the diagonal aids. I want a horse to come very well off of the inside leg before I even start to ask it to move off of the outside leg without having a lot of ground covering forward impulsion. [It is so easy for a small circle to turn into a dropped shoulder or a horse 'diving and falling into a turn-around.]
Each learning session has to build on the skills learned in the past sessions. When you skip one and don't do the proper preparation, you get either an inadequate response or it comes back to 'bite' you later.
So this horse needs to learn to get off that inside leg -- even if it takes a bat tapping it on that shoulder and it needs to learn to properly follow its nose.
It is late, I am tired, (it is still 90 degrees and was only 107 today) I am rambling and hope this is understandable. It got waaay to long and involved. Sorry OP if it is too complicated.