Also try some saddle ques like kissing or whatever ques you use to get him to canter in saddle
If you already hit behind him with a lunge whip don't be scared to smack him with it to tell him you mean business it is NOT abuse be cuase soon he will pick it up it a heartbeat
maybe swing the end of the lunge line towards him or her or use a lunge whip! good luck
I like to start canter myself because horses like to mimic you and at the same time say canter. When you've done that a couple of times your horse will learn your clues. Good luck! 😊