I actually have a draft paint cross as well who has a stubborn, lazy side like yours. She's only 3 and we are just getting over the same sort of issues. she will refuse to move forward and plant her feet, the only difference is that when pushed my girl will rear.
I started putting my mare on a lunge line with a rider, either I was on her or the one holding the line. When she looked like she was about to balk and act up the person on the ground would be there to get after her and ask her to move forward. Eventually she started to realize she wouldn't get away with it and now will listen to her rider.
I would see if you can get someone to help you on the ground. Your horse is choosing to ignore your cues. If he lunges well he might be happier to listen to someone on the ground, and if he really is confused it will be something he understands and can connect to the aids you are giving him. Either way he'll quickly realize it's easier just to listen to his rider.