Clamp him very firmly using your whole body; straddle him, with his backside pushed into a corner of a closed pen. Have your bent knees in front so you can grip the sides of his chest with your knees to prevent him jumping forward and put some of your weight pressing down along his body so he can't jump up. Use an elbow to keep his head pressed into your body while you work on the horn. I'm sure you know to check not to cut where the horn is warm, just the cold part ie with no blood supply. As you saw, try to cover his eye so horn dust doesn't get in - you can do this with the hand which is steadying the horn. If you are able to use loppers then there will be no dust.
You do need several more arms and legs than you were born with to do this. Is the horn just heading towards the face, or is it actually pressing against the bone? Ideally you should wait until you are better - in spite of animal welfare being important, your welfare is actually more important, so if you don't have the strength at the moment, then put it off for a week, unless the horn is actually growing into the bone or has gone through the skin.