As above, there are no antitoxins for clostridial disease, giving lambivac at this time would only make it harder for his already struggling immune system.
However, it sounds unlikely to be enterotoxaemia as the clostridial toxins would tend to cause much more rapid and severe signs. As he has been unwell for two weeks, but is still alive, I think you need to get your vet to see him.
My gut feeling as a vet from what you describe (but would be much more sensible to get a vet to do an examination and listen to the gut, feel the goats abdomen, take its temperature and often samples) is it may be floppy kid syndrome or another gut condition, possibly coccidiosis, which also cause diarrhoea and unwellness.
Your vet will be able to narrow down the problem and give medications to make him more comfortable passing faeces and stimulate the appetite.
The other absolutely vital thing is to make sure he is passing urine, as male sheep or goats that have been fed concentrates are at risk of bladder stones which can cause blockage and straining to urinate with small amounts or no urine produced. This is a life threatening emergency that will most frequently need surgery to save a life.