OK I’ll give you the correct training for this.
1, you need to take your horse out on the trail and climb hills. Every day. Up and down – Up and down – Up and down until you’re so sick of it you can’t go on. And you’ll still have to keep on. You are to do this at a walk!!!! Climb climb climb I learned this well over 30 years ago when I had my first young stallion with a huge neck and a weak hind end. I have since always climbed my horses and have always had super strong hind ends. And perfectly shaped. So – out on the trail with the two of you and start climbing those hills.
2, the neck. That’s flat work. Lots of long and low at a trot. Frequent tempo changes, every three strides from working to collected to working to collected. To this add half-halts and gait changes. Walk/trot trot/canter walk/canter. Frequent changes. Think ultimately four + on a 20 meter circle. But, to get a correct neck you have to use “long and low”. You need to ensure you engage his hind end as the hind end is what will power up the horse and push the force through the horse through the neck. I recommend some Dressage lessons for the two of you. Tell the Dressage trainer what you want, to fix the neck and that you need to learn how to get a correct long and low so that you can fill in the neck and then raise the neck for correct carriage for your classes. This is not upper level Dressage – this is regular Flat Work. A good trainer should know what you’re asking for.
I can promise you that this will fix your problems. Now it’s in your hand to decide if you want to actually fix your horse or do some silly fast fix which is no fix. Correct work and within months you will see your horse change.
Also, I want to point out that uphill trotting is shown in studies to be a sure way to wear out your horse and not actually recommended.
Climbing a hill at a walk is far more body building for them. I also highly recommend Power Walking your horse for 2 hours at least once each week. This is done on the trail. Power walking means you’re actively walking your horse forward for the two hours. Not for you to be chatting with your best barn buddy which the two of you are out on the trail ride. You are to actively make your horse move out and forward.
Good luck from someone with horses with great necks and hind ends.