An English saddle, arguably even moreso than a Western due to the surface area or lack there of, indeed requires proper fitting. At this point in your horse’s progression under saddle and his age/work rate, he can be considered not old but experienced : ) His body has likely changed, ‘specially where he was living in and out of irregular work and is now in steady activity. Your saddle (and your seat use) is KEY in his comfort. What fit initially might not fit so well now due to refitness. It is also possible that he had a pasture incident, however seemingly minour, that jammed up a hip or something. Chiro and/or massage is indicated here.
Coming back into work later in life after idleness can be a tricky thing. You might consider a massage as this can relax and sometimes pinpoint many structural issues. It can also support or rule out the need and effect of chiropractics.
Your horse is trying desperately to tell you that something is not right for him. I think you would be safe to start by looking into his saddle/back first, then teeth if they are due to be floated, and try to determine the source of his issue(s).
There are a whole bunch of reasons why he might be behaving like he is, I offer only the common and most assumed causes. You know the horse best, keep digging til you get to the source. Good luck, be careful and I hope this can be resolved with an uncomplicated solution for both of you : )
Sure there's right and wrong but mostly there's just a whole lotta different.