Is the bit comfortable? Does it suit his mouth conformation?
Does his saddle fit?
Are you a balanced rider, with a seat independant of the reins?
Is the horse on his forehand at all? Is he trained from the weight aids and is he physically able to carry himself?
All these things need to be considered as any one or more of them can make a horse lean on the bit
Saddle fits as recently had it checked and reflocked
teeth have been recently done
I have him in a french link snaffle and it fits him fine.
He was a riding school horse until I recently bought him.
I expect he may well be on his forehand then and therefore will naturally lean into the bit as it is hard work for him to engage his hindquarters and carry himself more behind. Also, he may well have developed leaning as a response to being pulled in the mouth as an aid instead of riders using their seat.
To progress will take time and schooling. Teach him to slow down, decrease pace and stop by using your seat only. Dont worry if it takes several metres at first, thats normal. Do not be tempted to use your reins when practising, otherwise he will be stopping because you are pulling on his mouth and not because he is using himself. Also do exercises to encourage engagement of the hindquarters. Make sure you dont tip forward as this will hinder his ability to stop leaning.
It takes years to teach a horse how to carry himself and some types/breeds find it easier than others. Cob types for example are built on their forehands more than others. You just have to be patient and consistant and he will stop leaning but it requires training! And never be tempted to pull back, it ruins their mouth and you will never be as strong as a horse!
Agree with Teddyt as above - plus lots and lots and lots lots etc etc of transitions. What also helped me was having an instructor ride my cob. She was able to teach him what she wanted & also then tell me how to do it properly - ie in the right way for him. It is still a work in progress lol
I agree with teddyt and Barneykins. Most horses that lean and/or are heavy are on their forehand and therefore not in self-balance.
Most of the time I would say that changing the bit is not going to solve the problem until the basic way of going has improved. I have successfully used a 'magic bit' (yes, that is its name) to encourage a horse to stop leaning. It is like a straight bar snaffle with a small port and small rings.