My daughter has a YB that went off feed at about 1 yr of age. It lasted for about 80+ days. Last year breeding season it went off for 110 days. It happens. Just monitor its weight and make sure your husbandry is correct. Try offering food about once every two weeks right now. It wont hurt since he's not eating anyways. As long as there is no illness he should start back up eventually. Patience, try not to worry, and good luck.