In the left lead, the foot falls are as follows...
left hind and right fore together
It is called left lead because typically people are tracking left when they canter on the left lead and, more anecdotal, the left fore appears to be "leading" or stepping out further than the right fore.
The pattern of footfalls is why you get that lovely 1,2,3, 1,2,3 rhythm at the canter. But the right hind is working very hard on the left lead as at one point during each stride it is bearing all the weight and it is also responsible for a great amount of thrust/impulsion during each stride.
So if he will do it and is sound doing it, but swaps out sometimes... and since you've had extensive vet checking done.. I would just speak to your vet and trainer about it and ask them to give you specific exercises to strengthen his hind end and particularly that right hind.
Good Luck :)