K.M.
Like Lisa said, it could be emotional. Don't accuse the girlfriend of doing something yet. Let your ex know that your son is getting sick when he comes home. Ask him about what he eats. It could be something as simple as reflux. For years my 6 year old has off and on had problems with food with tomato sauce. Sometimes he makes him sick, sometimes it doesn't. I hope you are wrong about the girlfriend, but as you investigate different possible causes don't dismiss poisoning. Stranger things have happened. Ask your son also, if he is sick while he's at Dad's house too.