Very good start. As mercedes said, you just need to get more control of the shoulder.
I would be leg yielding from the 3/4 line onto the outside track rather than through the middle of the arena. Going down the 3/4 to the outside track gives you a line to go by and helps you keep straight. Leg yield is the crossing over of the front and hind legs, but the body remains straight with a little flexion of the jaw. You have your boy with a very strong bend away from the direction of travel. You want his body straight and only his jaw flexed slightly in that direction. This has caused you to lose the shoulder to the outside.
To correct this, again as mercedes said, outside rein. So say you are leg yielding to the right, going down the 3/4 line heading back to the outside track. Come around the short side onto the 3/4 line, and ride dead straight for a few strides, then apply the aids for leg yield. Keep a solid outside rein contact and push him across with your inside leg. You should barely need any inside rein contact, only a touch to get the flexion. When he goes to overbend to the inside, half halt strongly on the outside rein and get him to momentarily 'stop', give a firmer inside leg aid to push him into the strong check on the outside rein to straighten him, then walk forward for a few strides before asking for a shallow leg yield again.
When I have horses that tend to overbend in leg yield I find it helps to walk straight for a few strides, then leg yield a few strides, then straight again for a few strides and so on. It keeps you and the horse thinking straight rather than sideways ;)