HI,I'm Dr Steve. I am sorry that you are having trouble with Grumpy.
When a herd animal such as a goat separates himself from the herd, it is not a good sign. SOme say that they are going off to die. I don't necessarily believe that, but it usually indicates that they are sick.
Having a reduced appetite canbe caused by many things. However with his relatively young age, the thigns I commonly see with these symptoms are severe worm infestations ( but it sounds like you have addressed that already and that is good) and bacterial infections that can cause a fever.
When a goat becomes feverish, they feel so poorly that they often are not interested in food, water or being social. They also usually become lethargic and depressed.
The first thing to do is to take his temperature using an inexpensive human digital rectal thermometer. If his temp is more than 39C, then he has a fever and probably an infection.
If he has a fever then he should be treated with antibiotics. I have had good success with using excenel or naxcel for infections in goats. Some people use penicillin, but I have seen a few goats who were allergic to penicillin and had fatal reactions, so I tend not to use penicillin.
If his fever is close to 40.5C,yo may want to have your vet come out and give him antibiotics IV and give flunixin as pain medicine. He can also check him for dehydration and give IV fluids if necessary.
If he has a fever, nce the fever is gone and the temp is back to normal (38.5C), then he may feel better and want to eat. In most cases it is difficult to know where the infection may have come from, but fortunately these antibiotics have a broad spectrum of activity , meaning they will kill a lot of different types of bacteria
As I mentioned, young goats are typically pretty resilient and the most common thing I see is infections. This should be checked out either by you or your vet and treated . The sooner it is checked out and treated the better the prognosis should be. I hope this has helped give you some options for what things could be done for him. If it has would you please rate my answer so I may receive credit. I will be online for 2 more hours and will check in the morning ( 7 hours from now) to see if you have more questions. Thanks for the questions.