Could it be maybe he has hurt his back , do you know if any of the vets in your area do chiropractic work, I had to have Karl adjusted when he was just 5 weeks old , he fell off the couch and wrench his neck, my vet is schooled in it but she has never had to adjust such a young pup but wow it was like he had instant relief , he also whined and cried out in pain when he moved but after she gave him a couple of adjustments and his crying stopped and he was back to normal puppy self the next day. Its worth a try but only if the vet knows what they are doing and it my be worth it to talk to them about a mild sedative , cause he is a fila lol. Also any hydro therapy places for animals near you, horses or dogs, that is great healing therapy, no stress on the joints, I know some vets are starting to carry their own hydro therapy treadmill in a box that gets filled with warm water so the dog can still walk but is buoyant enough to not feel their weight. But I would definitely take him in for maybe some x-rays in order to make sure there is not more serious issues happening like a slipped disk, and I would def restrict his movements, rest , only up and out to do his business and back to laydown.
I use Turmeel (sp?) its an all natural anti inflammatory with Arnica, and seemed to work on my cats and the dogs I have used it on, I learned about it a few years ago from my vet. You say he is downstairs and I take it you sleep upstairs? is he kenneled while he is downstairs?