Hugs! It's so bad when they aren't doing well. More hugs!
What kind of
milk did he drink?
What is the diet and minerals you are giving him? Is he getting salt? Note, salt and minerals are considered separate and different things with livestock.
Is he dark in colour? Black or brown?
My first instinct is something wrong with him not getting
enough Vit D. Either something about the milk that he isn't digesting the right kinds of fats, dark colour wool which will prevent Vit D absorption, or an imbalance in his regular minerals (too much of one, not enough of another).
It's also very much like early-mid stages SE (selenium) deficiency - but before you give him SE, it would be good to know where you are. An overdose of SE kills quickly, but SE deficiency takes several weeks/months to kill a sheep.
Minerals are really difficult because the pre-mix minerals aren't customized to different locations. I have to buy special minerals this time of year to supplement the regular mix.