Yup, get the vet. He is in pain. In fact I would go so far as to say do NOT get the chiropractor unless the vet OKs as it can make it worse, cancel the appointment and ask the chiro who they would recommend for a vet. Any chiro that says "oh you don't need a vet I can do an adjustment" (unless they are a veterinary chiropractor :P) is not someone I would consider hiring. A good veterinary chiro will probably still recommend a regular vet to do x-rays and such.
I'm assuming his issues are old age/arthritis though he may easily have others as well and really needs a check up if he's that bad (sounds it).
Unfortunately the things I would recommend are mostly expensive, so to be a little less specific I would look into a good joint supplement, an anti inflammatory, herbal/long term pain relief*. Some of these can overlap. Also look into management and make sure you're following all the protocol.. I would go ahead and do all those things, no reason not to. Just mention that when you talk to the vet so he/she can ok what you're doing.
If the vet thinks he needs more in the realm of pain relief you need to go prescription for something stronger with Previcox being the drug of choice for long term use.
He may also recommend injections (such as Adequan) or joint injections.
Unfortunately your horse sounds very severe. There could EASILY be something else going on and if he's that bad from arthritis or such he may be at the point where euthanasia would be the most humane option. I wonder where he is "stiff" on the front legs to where he can't walk if he's just very very lame on both fronts? Laminitis? Navicular?
Ultimately it's all just a guessing game. Please get the vet for your horses sake, and let us know how it goes!! I know it will be a haul to get there but he absolutely NEEDS it, either that or if you can't do what he needs put him to sleep and don't let him suffer.
I am curious if you have a video of him moving?