OK - it is not about strength, it is about timing!
The only time you can influence a horse's leg is when it is lifting off the ground.
So get into the habit of counting the hoof falls - then hold the outside rein to stop him turning in and kick or nudge with the inside leg as the inside rear hoof leaves the ground - he will step over towards the outside and stay on the track. This is leg yielding and they all know how to do it!
So, in walk feel the inside rear hip drop and kick. In trot, kick as you sit. In canter, 1 is the outside rear, kick on 2, the double leg stride, 3 is the leading leg.
Keep the rythm by saying 1, 2, 3, kick, 1, 2, 3, kick or up, kick, up, kick or 1, kick, 3, 1, kick, 3.