If your dog has anxiety and/or OCD, eating non-food items can be a symptom of his mental health. It's a disorder called pica. But unfortunately it can also be a behavior you unknowingly reinforced in the past.
I have an anxious dog myself and I noticed that chewing on things seems to be a coping mechanism for anxiety. My dog used to chew on furniture, his leash and blankets and he used to absolutely destroy his blankets and swallow big chunks of it. Of course we took the blanket away when he started chewing, but that only increased his anxiety and his chewing.
Maybe a similar thing happened between you and your dog. He started eating leaves, so you started taking the leaves away from him. He either learned that leaves are a very special and rare thing that needs to be swallowed as quickly as possible before you take them away, or he feels your own anxiety around leaves and gets more anxious and eats more leaves. It's a behavior that spiraled out of control.
Taking him to the bathroom on a leash only works for you if:
- You can stay calm and trust your dog not to eat an unhealthy amount of leaves.
- You don't have to be super vigilant and pull his leash every 5 seconds.
- Your dog can relax enough to use the bathroom.
I would try putting a muzzle on him in addition to the leash. If he accepts the muzzle well and it stops him from eating too many leaves, maybe you can switch to just using the muzzle without the leash.
I would also strongly recommend a behavior redirection training for him. In short: instead of punishing him for bad behavior (eating leaves), you redirect his interest to a good behavior. The easiest method is to offer him a treat that smells delicious. That can either be a commercially available dog treat he absolutely loves, or something like a hotdog wiener cut into small pieces.
You start by teaching him a new command that only means he gets to eat the treat. You say the word, you feed him a treat. Train this for 2 - 3 days until he associates the new word with getting a treat.
Then, whenever you go out with him, have a small bag of trats with you. Clear the immediate area of all leaves, but have one leaf lying out of his reach. When he notices the single leaf, call the new "treat" command. The goal is to have your dog looking at you instead of the leaf. Always give the treat to him, no matter how long it takes for him to react. Reward him by telling him what a good boy he is.
After a week of this training, you should be able to assess whether it works or not. If it works, you should also give him treat-commands when you walk with him and he sees leaves lying around. Always give him the treat after saying the "treat" command. Since the treat is more tasty, he should eventually redirect his attention and ignore the leaves.
Since you already have a therapist working with him, it's best to ask them for guidance and tips for the training at home. There are also videos available by professional dog trainers, like this one. But you have to be aware that what looks so easy there in 3 minutes will take you several weeks and months to accomplish.
This video about teaching a dog "leave it" is also helpful because the trainer explains the difference between punishing behavior ("leave it or something bad will happen") and redirecting behavior ("leave it because I offer you something better"). In case of the punishing "leave it", dogs may learn to just eat the thing they're supposed to leave alone faster before it's taken away or they're punished. Dogs are much more inclined to change their behavior with positive reinforcement than with punishment.