Best thing would be a FEC, if you have a microscope you can do it yourself, pictures online.
And the Famacha test, colour of goats eyelids against a chart shows whether anaemic.
Could be looking at Johnnes disease? Thinking of dramatic weight loss.
Do you get deer in the area/field? They could introduce problems.
I give mine Vitaferrin as a supplement if I think they need it, iron, vits and minerals, is a good pick me up.
Some goats could be just more susceptible than others.
Oh, check his teeth, as my girl got older, her teeth didnt meet the pad.
Hope you can sort things.