I would recommend practicing (having him rehearse) going outside. If he's okay indoors, but behaves wildly as soon as you go outdoors, that means going outdoors is a big deal to him. It shouldn't be. A well-adjusted dog won't be stimulated and amped and uncontrollable (and possibly anxious/fearful/reactive) when going outside.
Practice going outside before going outside, i.e. get his attention indoors (on a lead), open the door, hold his attention while door is open, close door. Once you can hold his attention at the open door, you'll start going outside, one or two steps, then turning around and going back inside. Slowly you should go further and further outside until you can hold his attention away from the building and have him start regarding you outside in general.
Again, the process above is at your dog's pace.
Harnesses are a great tool for building motivation, because the dog can be restrained and frustrated, thus building drive and desire. In this instance however, there really isn't a need for a harness or extra motivation to go outdoors. And continuing to use it will ensure that he pulls harder. You should probably switch to a collar. In fact, you may wish to begin leash pressure training with a slip lead. You can socialize him to the slip lead prior to using it, by having him wear it while feeding/training and taking it off when done. You can socialize him to the collar by practicing taking it on and off with food.
I love these tips! We will switch to a collar (he is used to wearing one as we sometimes walked him on collar too) - hopefully this is just a phase but I really like your tips on really taking it back to basics with him to get him back on track! I will try this and then hopefully it will help him ping back to the way he was just a couple weeks ago - thank you