In my experience, you don’t want to muscle a stallion. One, strong-arming them is a direct invitation for physical rebuke and stallions tend to think a whack is invitation to play — and two, constant physical correction desensitizes them and then you’re really in trouble.
My two cents: start working him in a rope halter, not a chain. Not sure what you are using now, but I don’t like chains because they don’t release pressure quick enough, desensitize the horse to pressure, and can overcorrect. I especially hate stallions in chains because IME the constant pressure makes them rude and bargy because they don’t care anymore, because the pressure never goes away. Rope halters release pressure as soon as you release, where chains get ‘caught’ in the metal you thread it through and sometimes don’t even release at all.
I know it might seem humiliating, but why not ask his mom to help you with it? I have tremendous respect for people who say “wait, I don’t know how to do that, can you show me how?” - it shows initiative but it also shows that you’re a critical thinker, and you can recognize your own short-comings and how to improve them. That’s much more useful to me than someone who just continues to go on oblivious to their environment or how they might not be handling a situation appropriately.
How are you asking him to do these things?
Some tips for teaching a horse to yield to pressure:
Start with backing up, provided the horse is not the type to rear. You ask the horse to stop, take a second to pat it, and then apply pressure with your hand to the chest, and with your other hand, to the lead – as soon as the horse drags a foot back, praise lavishly and rinse and repeat.
Don’t push - horses instinctively lean into pressure, so you need to be using a tap or poke to get the horse to move over until they do. Do these quickly - tap-tap-tap the flank and stop the second the horse’s barrel shifts away. For particularly bargy horses, I might not use a hand at all and instead twirl the leadrope until they give and move away.
I always use a voice command first. ‘Back’ for back up, ‘over’ for move over - followed by a gesture (no contact) - if that does not get a response, it is onto contact - a poke, or tap, until the horse moves over. It only takes a few sessions to install a reaction to gesture alone - for instance, ‘over’ , I’ll say “Over.” and move towards the horse (gesture 1) and place a palm out as if to push the horse over (gesture 2) - at this point, my horse knows to move over and almost always does before I need to contact him with a tap.
The goal is to condition the horse to respond before you add pressure. A lot of people think they need to touch the horse physically to get it to do something; no - working on getting the horse to respond to voice and gestures is, IMHO, safer and easier than always needing to physically poke or haul the horse’s head to get it to do something.
The other thing is… 90% of interacting with a stallion is doing so preemptively… you know that popular phrase… CONSTANT VIGILANCE! Well, it applies to stallions too - shut down behaviors before they have a chance to happen… extreme example… you ask him to stop and he starts to balk - that’s your cue he’s about to rear – rather than pull against him or try to get him to stop what he is doing, go to his flank and ask him to move over, or walk on, or whatever – or, better yet, work him in such a way that you are not enticing him to balk… either halt towards the gate, or keep the sessions short enough they don’t get to that point where they’re looking to fight over cooperate.
A less extreme example would be the nipping - it can be really hard to shut this down in a stallion, but I’ve found that smacking them does not stop the behavior as reliably as completely preventing it from happening… easier said than done, but most studs have a particular circumstance (something I call a behavioral routine, or a reliable habit demonstrated consistently in response to a specific stimuli) they like to get nippy over and you should critically go over when and why the stallion bites and look for ways to avoid that - IE some people hold the lead too close to the head, and studs nip then - try not to hold underneath his halter, and keep your arms closer to your body rather than out… another time I’ve found studs bitey is leading in from the paddock - doing zigzags, circles, and not going straight to the stall can help break the behavioral routine… and in the event the horse does nip, try to arrange it so that as he goes to nip you, he comes into abrupt contact with an elbow. Smacking after the behavior will not, in my experience, prevent the behavior from happening again - you have to make the actual behavior (in this case, biting) uninviting, not the reaction to the behavior.
AKA - with stallions, it is much more about prevention than reaction -if you are reacting to an unwanted behavior with them, IMHO, you’re too late.