Sounds to me like a pretty classic set of behavior issues related to a combination of saddle fit/back pain coupled with chiropractic issues. A horse that's in back pain will tend to try to stretch more under saddle, hence the diving head down. Some horse buck, but others do not and it's a sure fire way for me to know when my one gelding needs an adjustment. He will pull the reins out of my hand and dive down onto the bit and just want to stretch and pull.
However, chiropractic issues are also coupled with saddle fit issues. You said your horse had some time off, which likely also means that he changed shape since you last fit the saddle to him. Lack of muscle mass, increase in weight, change in shape due to lack of work ALL can contribute to a change in the way the saddle fits. It's entirely possible that the saddle is exacerbating or even causing the likely chiropractic issues.
Find a good chiro in your area, and have them out ASAP, and also find someone who can help with saddle fit. From there, build up to walking and yielding to leg pressure, and some longe work to bring back fitness, and then incorporate more "regular" workouts as your horse progresses. Also please note, that having a chiro but not addressing the cause of the chiropractic issues will not solve the problem - but just offer temporary relief.