Is there any way you can contact the previous owners to ask what they did? Since he came with clipped ears, you know he will stand still somehow.
Since he is so terrified now, I'm sure they didn't do anything worth doing again. o_O
How long did you have him before you tried to shove scary, noisy, vibrating machines in his ears? It's entirely likely that you pushed him too far before he was ready and now he just doesn't trust you. If you gave him ample time to create a bond with you before you tried clipping his ears, his previous owners probably went about things poorly, and now he's scared of the clippers.
Either way, at this point, you need to step back and re-introduce the clippers as a positive thing. Don't start with his ears, start at his withers, his butt, his fetlocks, wherever he is most comfortable with them, and SLOWLY work them closer to his ears. Say you start at the withers. Keep the clippers turned off and rub his withers or shoulders. If he's okay with that, bring them higher up his neck. If he starts getting antsy, just hold them there and wait until he regains his cool, then take them away. Put them down, take him into the yard and let him hand graze for a minute or two before you go back and try again, starting at the withers and going slowly higher until he starts to get upset, and when he calms down, reward him by taking them away. Once you get to his ears and you can rub the still turned off clippers around them, go back to his withers and turn them on, repeating the whole process. It will take time, and I'm glad to hear your giving yourself until May. If you do one or two twenty minute sessions a day, he should be clip-able in no time. Good luck. =]