I had this happen with my ridgebacks in the past. For them they were bored of eating the same thing for so long. Have you tried toppers on his food like yogurt or green beans? However, on the flip side there could be a medical reason as to why he isn't eating as well. Is he drinking as he normally would? Is he doing his business as normal or is it less frequent and is his poo runny now? Have there been any changes in your environment?