Ataxia is a lack of balance, not just trouble moving.
What you are describing, Wren, could be a gait issue associated with pain but with it getting worse I would be very worried that it is neurological in nature. I would strongly recommend that you have a vet out as an emergency visit to see your horse.
If this is neurological, then it could quickly progress to a situation where your horse is dangerous to himself and those around him due to an inability to control his own movements and balance enough to stay upright. Until your vet arrives, I would recommend that you move your horse into an area where there are no sharp protruding objects, he has enough room to get up if he falls and where he can't get his legs caught up under need a fence rail, etc if he does fall. Also advise that no one handles your horse until after a vet exam--both for his safety and theirs.
If at all possible, you should take your horse's temperature to see if there is any fever associated with these symptoms. And think about things that have occured in the days prior to the onset of symptoms. Has he been hauled? Had any sort of accident? Has he had or been around any horses with an upper respiratory infection in the last couple of weeks or been around any new horses?
Licensed Veterinary Technician