He doesn't need to be taught anything. You do.
If poking him in the side doesn't help, well... why do you keep doing it? It isn't going to magically start working one day. You do what you need to do to get a reaction. Sounds like you need to keep a crop in your pocket. You don't need to "yell" at him. He doesn't actually speak English... He does, however, speak "horse". Any what would the lead horse do if he got shoved by your gelding? I'm thinking he wouldn't whine. He'd simply bite or kick. Or shove, but I don't think you have 1200lbs to throw back at him.
I'd personally stop using crossties. Crossties say "hey, my horse isn't trained to stand still". I'd get a rope halter, leave the lead on, then turn him out in the arena and let him figure out how to stand still. Every time he takes a step, move him back. Don't say anything. Just keep moving him for however long you need to. Eventually you can leave the area and he'll stay put, and even if he walks, he'll just step on his line.
Anywho, I'd suggest switching to one crosstie and moving him back whenever you need to. If he shoves you though, beat him back.