You mentioned he was stumbling about, is he limping or having trouble balancing, or both? Niacin or other vitamin deficiency can be the most likely cause of that and weight loss, but it could also be other issues like an infection or coccidia that could be causing malabsorption.
If it’s coccidia in the lower digestive tract you’ll see red in the droppings, if the droppings are black it’s in the upper tract.
How is his overall behavior? Does he seem quieter and less active, is he sitting around a lot? Also has his appearance changed?
If you could post a picture of his droppings that would be great, I’m not the best at judging beyond some obvious things but I know others here are.
I came across it on an older thread by @casportpony . Can you help please?
He is back to the weight he should be and getting around great. Though he has taken up a habit I don't like. Biting.
He grabs my clothes and shoes and hangs on. I guess he expects me to keep having treats in my pocket. 
He also likes one of my goats. 
