You are using you leg to push his hip to the inside, correct?
My gelding had this same trouble.
If he knows forehand turns, ask him for the departure, if he picks up the wrong lead immediately stop him and punish his hip by kicking his hip to the inside and then IMMEDIATELY pick up the lope again. Smack his hip with the reins or crop if you have to, just make sure its a negative thing for him. Then when he picks up the lead, allow him to go into cruise and pet him some.
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.