Sam is a 2 month old baby boy who was born at term by normal vaginal delivery and is being exclusively formula fed. Over the past 6 weeks, his parents are concerned about his faltering growth and that he often cries and appears to be distressed after feeds. His eczema is poorly controlled despite good compliance with emollients and topical steroids. Sam has also been having loose stools since birth mixed with mucus, and his parents are concerned about redness around his anus. He is not vomiting and there is no blood in his stool.
What is the most likely diagnosis?