Gill, Beau went through something similar when he was bitten on his back - off his food, high temperature, vomiting, no bounce in his legs... our vet at the time didn't spot the damage and assumed he had fallen badly, but I mentioned there had been a wet patch on his fur which might suggest he had been fighting.
Turns out he had a massive abscess on his back that was pressing into the spine and knocking his nerve out of kilter. Once discovered the abscess needed to be squeezed which was excruciating for him ( he bit me through to the bone, and the hot green gunk hit the vet in the eye) but the moment it was done he started to improve.
I hope there's as simple a solution to Lupin's issues, def get the vet to check his spine for any hot spots, as it does sound exactly the same symptoms, Beau was even the same age, just two.