My son threw up twice yesterday and before doing so i noticed that his lips had lost their colour and he appeared very weak. is this a concern? thanks

Not likely: Have you heard the expression, 'green around the gills'? It refers to a nauseated person's appearance most people have pallor and weakness when nauseous or vomiting. If vomiting persists, urine output is diminished, he remains lethargic, etc.-see your doctor. He might be dehydrated or have other issues compounding the vomiting. Hope he's feeling better today.