Unfortunately a lot of gelded horses change after the surgery. I had a stud named Sue I gelded and he turned totally aggressive and I had to sell him because I couldn't get near him. I had the lunge whip out and had to snap it all around him because he was lunging at me and kicking me. He nailed me right in the hip once and after that....Good-bye, Sue. :( I sent him to a rehab facility for horses (Unwanted, untrainable, or abused) because I couldn't handle him. He started doing better but is still agressive and only like one person, and that's certainly not me.
Just show him love and spend time with him, desensitizing him to the lead and teach him that when he relaxes, he gets praised and loved on. Be his herd leader and friend for a little while instead of his ride and his owner. I wish I could be more help but I can't :(
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.