Firstly, give him a flukicide, not a combination fluke / wormer. Even then, bluntly, in his condition, and on top of such a worming dose, it might kill him. But if it
is fluke, by the sound of it, you need to treat aggressively anyway. So the concern is that it's not fluke, and the meds kill him. If the alternative is Johnnes, he won't recover anyway
.
There are some other worms, which may or may not be treated by the treatment you gave him. For instance, coccidiosis (though adult sheep are normally immune), or barber pole worm (which I have read about but not experienced.)
And as others have said, there could be poisoning. Or other digestive upset.
Might be worth phoning the vet to see if they help you do a diagnosis, or maybe offer a FEC or something, or advise on best option flukicide if they think that's the best course of action.
Again, good luck