If it's not pain related how I taught my mini to lift her feet as she was a cow about it I would grab a dressage whip and tap the ouside or inside which ever worked and asked her to lift and when she did I would stop the tapping and praise her and now she is fantastic and if she doesn't want to lift I just tap the outside with my hand and say lift and she'll do what's asked.
Look for a pain issue. I have delt with this lots with rescues. 99% it's pain related
Pat his shoulder, run your hand down down to his fetlock, give a little tug and push your weight on to his shoulder. When he lifts just hold on, he'll soon gather you won't let go. Plus practice makes perfect
Pat down the leg so he knows your there a give a firm grip and the way down the leg and squeeze tightly along the cannon