Absolutely it will help him. Two of my babies were doing that when going out on trail, and you're right - smacking does very little.
However, it's a respect issue, so I go back and do all of the natural horsemanship exercises.
Then, I attack the problem: I have the mentality that I must NEVER let them get away with it or they will learn to do it more and more. Therefore, I almost always get off. Why put myself in danger, when I can just lead them over? I usually ride in a mecate rein or Indian hackamore so that it's safe to do groundwork. One of the babies would follow me right away once I got off, so I would usually get off between 3 and 4 times per ride to lead her over places she didn't like. I did usually try applying pressure first. Then when that didn't work, I'd hop off, back her up to show her I wasn't happy, and then led her. Then hopped back on 10 ft later. It was good for her because she's 4 and flighty but stands super quietly for mounting.
I really needed the hackamore for the second baby- sometimes leading wouldn't work. Then, I'd yield her hindquarters over and over again on the ground and back her up (exercises that I perfected in the ring first) and eventually she'd agree to walk forward.
But if you've already ridden through some naps, you shouldn't even have to get off - in fact, your horse is huge, so it would be really hard. What I would do is apply these same concepts when he refuses: make it HIS idea to walk forward by making him yield his hindquarters under saddle when he naps. Then, give him a chance by facing him head in the direction you want to go and applying some pressure. If he takes even a HALF step forward, reward intensely, and let him rest for a second before resuming. It's important to tell yourself that you will take as much time as you need, one hour, two hours, just doing this, but that you will never give up.
Also, ask for just a little bit at a time. I started just asking these babies to go on 5 min hacks, then 10 min, and then progressed to 30 hour and eventually 45 min. Doing this exercise for a whole 3 mile hack would have stressed them out. I had pretty low standards at first but made sure that I was the the one to decide when to go home.
It's invaluable to push your horse's buttons in the ring, first, too, where you feel the safest. Many riders have trouble doing this and wait until they have to, but usually when they have to isn't the safest situation. So, grab a tarp or something scary, and practice this exercise in the ring until he knows the whole hindquarters and pressure thing and stands on the tarp. I did that with the first baby (not the second, and I had SO much more trouble with her) and it helped so much!
Sorry for the novel, but this is one of my LEAST favorite issues so I hope that this advice helps a little! You can call it "Natural Horsemanship" but it's just basic horse training - try to make it HIS idea, reward the slightest try, make sure you have good timing in your reward and application of pressure, etc, etc.
You horse sounds beautiful btw - good luck with him! Do you have any pictures?