Always over exaggerate the flexion to the leg you want, and leg yield away from the wall to get the bend and make it really obvious that you want the counter lead. Then, with the new outside rein, round... inside rein, flex... and lighten with the inside rein once you have the bend. Then pick up the transition
put the horse a litle bite bended to the wall and then apply the ouside leg and the inside leg in their correct place to start counter canter. Don't forget that this exersercise is hard so don't let the horse "fall" into trot.
The best way imo is to start on the right lead and then change rein to get on the counter one. Try to relax and cut off the corners to keep it simple. When he catches on you can try to make the transition from trot to the counter canter. Inside rein, leg and you can use a whip to tap the inside leg to encourage the counter lead. If he still doesn't get it, you can try to make the transition straight across the arena, slightly bending to the side where you don't want to go, to help get the right (in this case counter) canter. Good luck :)
Maybe it would be an idea to pick up true canter then change rein over the diagonal and do short stretches of counter canter on the straight to get him used to the idea of cantering on 'incorrect' lead. Generally they're taught to go on the right lead so even if you're asking correctly from walk/trot he'll be confused as a result. Transition work off the wall on straight lines between the 2 leads (true trot counter trot true before having to turn) can also help establish that you want the lead you ask for, not the lead he wants to give. Make sure to be doubly clear about the respective leg positions with slight bending at the poll towards the inside of the lead you want.