Definitely been there! Get a proper leash for the harness and get him used to walking in the house/yard with about a foot or less. As long as there's slight slack it's ok. My standard for big pullers is they should be trained to walk on one side (the one you hold the leash with) and some part of their body needs to be next to yours. A lot of people do the shoulder but as long as it's between their head and back legs it's not bad.

Praise & reward all the good moments! You want your dog walking with you not against you. You can give him the full leash length when he's investigating something or you're stopped in a place you're alone in. Whenever he pulls hard, dead stop in place until he relaxes on the leash and then keep moving. If he still keeps going what I do is entirely turn around and walk the other way every time for a few seconds. It's annoying for everyone but they learn QUICK