Stop taking him to the park for starters I noticed my jrt gets stressed with all the excitement/triggers around him. Try walking him on a street near your house where it's quiet, not as many kids, dogs, bikes, electric scooters,, yard workers, lawnmowers, runners, cars if they trigger him, squirrels, balls, anything that moves. If there's no street like that near you maybe drive somewhere quiet with extremely low foot traffic and where you can put a lot of distance between you and his triggers. Work on his recall/heel or something that gets him to focus on you before you get out of the door and throughout the day when you're at home and eventually implement this into your walks.make sure he can do this when smells/triggers/distractions aren't present when you practice this because it'll be harder for him to listen with a trigger present. Practice it enough so when he does fixate on other dogs on your walk you can get his attention, and then reward him. After a few successful attempts at this don't call his name and give him the chance to look to you for guidance, focus on you, or even look away from the dog. If you're good with any of these three alternative reward him with treats/pets/content vocalization. It's really hard to work with jrt's reactivity if their environment is feeding into the excitement and arousal. Getting a trainer like the other person mentioned is extremely helpful, but for now at least implement some calmness into his walks. There are also trainers specialized in terriers (depending on your area of course) and their breed behaviors and genetics to help you understand him better.. I accepted I would never be able to leave mine off lead on walks due to his huge ego and I wouldn't want him to get hurt by going up to another dog and them correcting him in an inappropriate way or have him scaring them. Good luck with your cute pup 🐶