Your horse's weaker hind leg is the leg on his soft side. There's nothing wrong with your horse! Almost every horse has a weaker hind leg because few horses are ambidextrous. The weaker leg is the one on your horse's "soft" or hollow side. The stronger one is on his stiff side.
The weak hind leg doesn't step directly underneath your horse's body. Your horse displaces it slightly to the side to avoid carrying weight with it. On the other hand, the hind leg on the stiff side carries more weight.
The issue here goes back to that old saying "use makes the muscle". If you don't work on strengthening the weaker leg, that leg gets weaker, and the stronger one gets stronger.
This can lead to all kinds of problems down the road like having an uneven contact with the bit, uneven lengthenings, and difficulty doing lateral work in one direction.
Avoiding carrying weight with the weaker hind leg can be very subtle. Often your horse will put this leg only an inch or so to the side. An observant ground person can tell you which hind leg your horse is "unloading". Walk and trot straight away from her. Then change direction and do the same. If your horse's left hind is weaker than his right hind, he'll carry it slightly to the left regardless of which direction you're going.
Since this evasion can be subtle, your benignly antagonistic correction can be subtle as well. The solution is to ask your horse's left hind leg to do a little "weight-lifting". Do this by moving his hindquarters an inch or so to the right so this leg has to step under his body. Ask for this position in both directions on all lines and curves. This will give his weaker hind leg an opportunity to get stronger. One word of caution here. Since you know this leg is weaker, be sure you give your horse lots of walk breaks so he can relax his muscles. There's a fine line between strengthening muscles and making them sore.
If your horse is a bit more educated, you can do the same sort of exercise by always placing him in a very slight shoulder-fore or renvers position when you track to the left. When you track to the right, put him in a very slight haunches-in position. Every position should place his left hind leg a hair to the inside of his left front leg. Once again, a displacement of an inch or two is more than enough to do the job.
A Happy Horse