Honestly, if you're not showing him in halter, don't even put him through it. You have to religiously sweat a neck to get any results, and to maintain results, you must keep sweating them every day.
With his thick neck, he'd need the max sweat time, plus a rigorous work out on a line to get him hot enough to his neck will sweat, and will continue sweating for two hours(max. Time). During these hours, he needs to stand, tied, in a draft-free area (again, keeping him HOT and sweating, and from getting him chilled).
I know some hardcore halter people sweat for up to three and a half hours.
Then you have to worry about skin and hair care. You're going to need to rinse him off with water, completely after you sweat him - or he's going to get itchy and irritated. Then you need a leave-in conditioner to keep the hair and skin from getting dry and brittle.
Sweating has great results, but with great results comes great effort and a lot of hard work that just isn't worth it to you or your horse unless you're showing halter every weekend.
So do you think just regular weight loss will help his neck? Because to me he looks a lot better than what he did before, I just can't tell if his neck is getting skinnier.
Here is when I first got him
You've done a great job getting him in better condition. I would work him on the lunge with side reins and under saddle with his head long and low, driving from behind. He has overdeveloped muscles on the underside of his neck, making it look even thicker. If you can get him using his hind end, back, and neck correctly, you'll see more muscle in the rear, the belly pulled up, and his neck look longer and slimmer.
He doesn't have to go around with his head permanently low, you just have to work him enough that way to encourage proper muscle development. Don't force his head down though, as that will only compound your problem. You want to encourage his head low while asking him to drive his hind legs under himself.
My mare had similar conformation, though her neck wasn't quite as thick.
She went from this:
(horse in the front)
To this, with proper riding and a diet .
(she was sniffing pooh piles while I was free-lunging her, lol)