First, find out what he is leaning on you for
A horse leans on the bit for a reason, and the commonest is balance: the rein has become a prop, and as long as it stays fixed and strong he will happily rest his front end on it. Riders create the other half of the problem, because a hand that holds rather than gives teaches the horse that contact is a wall, and he leans on the wall. But new, one-sided or suddenly worse leaning is a physical question before it is a schooling one: teeth with sharp points, a bit that pinches or sits wrong, a sore back or a horse protecting a limb will all dump weight into your hands. If the leaning arrived with head tossing, an open mouth, tilting or reluctance to turn one way, stop schooling the symptom and have the teeth, bit fit and back checked; your vet and a qualified saddle fitter between them will rule pain in or out.
Give him nothing to lean against
The core fix is simple to say and takes feel to do: the moment he leans, soften briefly and clearly, so the prop vanishes, then retake a light, elastic contact when he is carrying himself. A half-halt is exactly that in miniature: a momentary check through your seat and hand, released the instant he responds. What never works is the long pull. He has a neck built for leaning against things and far more strength than your arms, and a pulling match just teaches him the rein really is furniture. Keep your elbows soft and following so the contact breathes with his head, and reward self-carriage within a stride or two of it appearing, because the release is the sentence he remembers from the whole paragraph.
Exercises that put the weight back where it belongs
Transitions are the master tool here, because a horse cannot lean and change gait at the same moment. Ride dozens of them in a session — walk to trot, trot to walk, trot to a square halt — and make each one from your seat first and the reins second. Circles and changes of rein rebalance him endlessly: a large circle asks the inside hind to step under, and a change across the diagonal resets the lean before it sets. Leg-yield is built for this problem, moving the horse sideways off your inside leg so the hindquarters step under the body and the forehand lightens without his mouth being touched at all. Ground poles in trot teach the same lesson through his feet: he must organise his own balance to step over, pole by pole, and most horses come off a line of poles noticeably lighter in the hand.
Shape the session, watch the markers
School leaning in short, clear sessions rather than long arguments: twenty to thirty minutes of transitions and figures with walk breaks on a long rein, several times a week, beats one weekly battle. Expect the markers to be quiet ones — transitions that need less hand, a halt where he stands over his own legs instead of parked on yours, a trot where you can give both reins forward for three strides and nothing changes. Progress is rarely straight: a tired, young or unfit horse leans more as the session goes on, which is your signal to finish rather than school harder. Fitness is part of the cure, because self-carriage is athletic work, and asking for it longer than his body can supply just re-teaches the lean. To keep every rider on the same page, the schooling exercises and how he responded can be set as shared duties in KinsLe Care Teams, so the next person who rides him continues the transitions work instead of unknowingly holding him up.