Every time your horse leans on the reins or the lead rope or on the handler see it as an opportunity.
Don’t just hold and teach him that it is ok to lean.
Don’t give to him and reward him for leaning.
Don’t punish him and make him too frightened to give you a soft feel when you want it. 
Don’t hold him tight and make him feel trapped so that he feels like he has to lean or pull.
Don’t avoid his leaning
by staying out of his way or letting him teach you.
On the ground or while riding, don’t avoid teaching him his responsibility to not pull or push on you.
Avoid making it something he must endure every time you handle him for the rest of his life. That is neither comfortable for the rider or fair on the horse.
You could just hold a little more and release when he gives you a soft feel.
Maybe move his feet a little until he stops leaning and then let him stand as a reward.
Also, if he is leaning hard enough, follow him and maintain that contact until he stops or steps forward and then release.
You could use your presence and focus to help him understand what to do to gain that release.
Try to set the situation up again a few times so that he has a chance to properly learn how to avoid being in this conflict.
You could be totally consistent about this so that he learns to have a conflict free time every time he is with you.
Find some help with this if you don’t know what to do.
You could teach him in a subtle way at a stand-still how to respond to a soft feel so that he understands it Giving your horse a nice life means not shirking your responsibilities here. The more you allow him to learn that leaning is ok the more he will have to endure the conflict in the future.
Often the behaviour that causes the horse and rider the most discomfort “is not” caused by anything sinister. It is often caused by unwittingly teaching the horse to respond the wrong way to things.
Remember every time your horse leans you are presented with another opportunity to make his life more comfortable in the future.
