Behind the bit troubles

My gelding just started coming back into work after health issues (heart and hind splint) and while I'm much happier about his temperment this year and willingness to move off my leg - the bugger evades the bit by ducking behind the vertical. I ride in a crank noseband and eggbutt snaffle and have loosened the crank (which I mention because it's new this year for him, versus his old regular cavesson). He is also shaking his neck a lot during and after the ride, which he's never done.

Now before everyone says push him into the contact and drive his impulsion. It's not working. He has impulsion in excess! I use demi-arrets to some help. I've decided to ride him long and low with light contact, which he'll do, for the time being before my next lesson (also first lesson of the season).

He does have a stiff right neck (has for a year since I got him) and find he ducks behind the vertical 'mostly' in bends that ask him to stretch and round his muscles. So I do carrot stretches, give him a message before and after the ride and always cold rub water/linament after the ride.

Any advice or suggestions?

What happens if you just toss him a ton of slack in the reins?

He travels with his nose to the ground or without any lateral bend.

I have a couple of young horses that I ride/train that go behind vertical when I start asking them to do hard work and they get tired. What has worked wonders for them is gently bumping them back up with both reins or just one, and riding them with the feeling of them being above the bit. Usually when I feel like they are slightly above the bit is when they are, in fact, just perfectly vertical. And then a good exercise is to leg yield in the trot, transition to walk and continue leg yield through the transition and vice versa.
I would not throw away the contact, since you want them to be through and powerful from behind into a nice light contact, thus making for a balanced, happy horse. So keep the contact, do minimal suppling when possible, and keep the conection soft through your elbows. And definitely reward your horse when he does the right thing! If you don't have mirrors to constantly check and see where he is vertical-wise, try to have a person watching you so you can keep tabs on it.

Bare in mind that contact comes from strength in the hind legs and back. If the horse is only just coming back into work he's not going to be strong enough to maintain a contact and stay in front of the vertical.
I would be more worried about getting him back to being sensitive off all basic aids first, and then focussing on getting the hind legs moving and stepping under your centre of gravity. Once he's going forward and really using those hind legs, the contact will start to improve.

Please also bare in mind that impulsion does not mean going fast. This is a VERY common mistake people make particularly when riding tb's. They think that the horse is in front of their leg because when they put their leg on, the horse leaps away, and think the horse is forward and engaged, because it keeps running on. This is not engagement and sensitivity to the leg, this is a horse that is running away from pressure to evade and is probably on the forehand. A horse travelling with impulsion will push off from the hind legs, which will track under the rider's centre of gravity.

Thanks, IslandWave, I started this the other day and have been doing lots of it since. It's definitely an exercise I am finding beneficial.

Kayty, I couldn't agree more about fast vs impulsion. The original post is correct however, he is definitely moving with impulsion (confirmed by third-party just to be sure) when he ducks behind the vertical. Don't get me wrong, he will try to be evasive by coming onto the forehand still occasionally, especially in the warmup but the bit troubles continue still with impulsion. I think his problem lies with his nerve-damaged neck and not having the muscle support (like you said just getting back into work) to laterally bend, since the most problems occur on the left-rein circles and into the corners.