Reduced Appetite in fifteen weeks old baby

Patient: Hi, My son is 15 weeks old. He is exclusively breast fed and is not under any medication. His birth weight was 3.5 kg and he now weighs 7.2 kgs. His urine output and stools are normal but for the past one week he has drastically reduced his feeds. Also, he has started spitting large quantities of milk after every feed. My paed advised to try Bonnisan syrup. Should I be concerned about GERD or any other reflux problems? Would Bonnisan help and is it safe for a 15 weeks baby? I used to feed him for every one hour, now he feeds for once in 2 to 3 hours that too for a 3 to 5 mins.