I can just imagine him. I disagree that because he is doing these evasions and has been behaving in this manner that he is "bad". There are very, very few horses that have a "bad bone in their body'. Very few. I bet he is actually a doll. But, his niceness can mean that people let him get away with too much, and a hrose that size can not be allowed to do that. It is ultimately unsafe. Ok?
So, I think that the small rears are him tantruming and you'll have to push him through and past that. Ignore it in the sense that you dont' take it as a personal insult. You continue asking for what you are asking for and do not allow him to dissuade you from that path by alternate responses to your "go forward, now". Emphasize the NOW a bit more, and do it a bit more prompltly. If you stay in the "gray" area, you allow him to think that arguing MIGHT get him somewhere, so he'll think that to continue trying might work, so he'll UP the ante. Cut that off pretty quickly. I bet you anything, he'll be surprised, maybe jump a bit, then go right on to work for you and forget entirely his plan to make you give up the idea of work.
The barging down on the rein should also be nipped in the bud. Use a firm hold on ONE rein, not two, to resist him (he cannot pull so hard against one, as opposed to how he can against two), and the second he goes to barge downward, hold firmly on one rein, lifting upward a bit, and put a firm leg on and get him to leap forward. Do not bop him in the mouth, and if he trots forward, go with him for a few steps, then GO RIGHT BACK TO ASKING FOR WHATEVER (TROT, WALK, CANTER) THAT YOU WERE ASKING FOR BEFORE, AS IF NOTHING HAPPENED.