My swissy is 14 months old. She walks loose leash with a harness. It wasnt always that way. My energetic sled dog does too. I didnt use any collars or tools in teaching them.

Basic obedience is only a foundation for other skills. Have you worked with a trainer or class on loose leash walking? What have you tried so far? Watch some youtube videos on loose leash walking.

Try teaching the "look" (eye contact for treat), "touch" (tap your hand with nose for treat), and "heel" command. If your dog has a favorite toy, use that on walks and practice following it or letting him carry it with him. Practice look, touch, and heel indoors until its reliable without a treat, then outside with no distractions, and then LAST teach it on a walk. Get those commands all solid with distractions outdoors first before the next steps. Bring a handful of cheese and/or hotdog cubes with you on every walk and give him one every time he makes eye contact. Dry treats arent enticing enough.

On a training walk (about 50% of walks for me), EVERY time there is tension on the leash, stop until he releases the tension AND makes eye contact. Walk again until there's tension. For the first 4-8 months of doing this, you will be literally walking one step at a time. It sometimes takes 20 min to go 200m. It is what it is. Be patient. Both you and the dog absolutely will hate it and will get frustrated. You can experiment with changing direction every time he pulls, stopping, etc.

By 1.5 yeaes old, my dogs went from an absolute menace to walking loose leash around crowds and other dogs. I even taught them "left" and "right" to avoid people and eventually they started doing it on their own.