puffy eye / Great Pyrenees dog

Hello, I need your help with identifying what my dog has . My dog is 13 months old. Since around 2 months ago we noticed that his left eye's sclera is swollen or puffy. We've been giving him antihistamines and putting colirio in his eye ,after a visit to a vet cause they told us it might be allergies. He was okay for a few days and then it happened again. We gave him again the same treatment and he was okay for a few days but then it happened again and it keeps going like that till now.  He is also loosing a lot of weight although he is eating good . Also he isn't very energetic . He used to eat poop before that ( i don't know if it has to do anything with that ) Do you know what can it be ?

Hello! - and I'm afraid that I'm worried.  Losing weight with a good appetite is a concerning combination.  Diabetes, some forms of liver disease, cancer and various other hormonal disease can also present like this.   Diabetes for example, is a serious condition and is often life threatening when first diagnosed.   Eyes are generally thought to be emergencies until proven otherwise, as so many ocular conditions can result in loss of sight if neglected.  Therefore, we would strongly recommend calling your local or emergency vet for triage today.  It could well be that they need to examine your dog in a very short time-frame.  Best of luck and please let us know how you get on.