My first recommendation is that you speak with his pediatrician as soon as possible, Claire. The doctor will be able to determine whether your son has a gastrointestinal disorder, which could cause his â€œbad belly." He may also be anxious about the birth of his new sibling.
Another question we always ask is if he looks like he's losing weight. It has been my experience that some children who won't eat a traditional meal are actually snacking throughout the day unbeknownst to Mom. A 5-year-old is certainly able to get into the refrigerator and cupboards on his own. To ease your concerns, and to make sure he's not actually sick, please call the doctor.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.