That is another question I had, how do I get him to lower is head/neck and use his back end?
I think Luvtoride made a great suggestion about that. Lunging him with side reins will encourage him to stretch down and use his back. If you've never used side reins before, here's a basic idea of what to do with them:
First you'll need to get your horse tacked up. If you happen to have a surcingle, you can use that - if not, you can get one for about $25 from Dover, and possibly less off of eBay. Of course, you also don't have to buy that - you can always just tack your horse up with his saddle pad, saddle, and girth, and the side reins will attach around the billets of the saddle. You will also need to bridle your horse, and remove the reins. (If he wears any kind of curb bit, you'll also want to leave off the curb chain).
As far as buying your side reins (if you don't have any). I prefer the ones with donuts, like these ones.
Either way, once you have your horse's saddle or surcingle, and bridle on, grab your lunge line, whip, and side reins and head to your arena (or any flat, well-drained area). The side reins hook from the bit (the back of the snap should be facing out) to the surcingle loops or underneath the billets of the saddle, and then comes back on itself to buckle. You'll want to start off using them on the longest setting, so your horse can get used to the feeling of them. After he's trotted three or four circles around you with them on the longest setting, you can move them up one or two more, so that when he raises his head, the side reins cause contact on the bit, and the contact goes away when he stretches down. In that way, it should quickly become clear to him that it is easier and more comfortable to reach down, than to resist the pressure from the side reins.
Oh gee! You shouldnt use side reins if you don't have a good idea on how to use them, rookies can make horrible mistakes, It would be perfured to get a lesson from someone rather informed on how to lunge a horse in a chambon they are simply amazing! If you like I could get a video on how to make a homemade cambon and how to use it?
See how its getting him to use his whole topline, its great for toning and building muscle it also lightens the rein aids and teaches the horse that rein pressure means head down
Also NEVER use the side reins with dougnuts the rubber rings on them bounce and jab the horse in the mouth OUCH!
If you have to use some use plain ones maybe with a rubber insert, but I wouldnt use them without instruction :)
Also I could make a quick video on how to lunge a horse if you like, a basics one?
I think if you just keep riding him it will go away.. his neck will probably still be big because it looks like that might be his build but it might go away. My two mares that I show constantly.. when I first got them there necks were HUGE.. but after getting them back inshape it went away.