My four year old son has lost a lot of weight and is not eating well. He is otherwise very active and playful. Should I take him to the doctor?

I'm a believer of asking your doctor anything - whether I think it's a small or big issue. Weight loss is certainly cause for concern so I wouldn't hesitate to ask my doctor about it. But in the meantime, you can see for yourself if your son's weight is still within the acceptable range. I recommend checking the WHO child growth standards chart. Here's a link to the chart for boys his age range: http://www.who.int/childgrowth/standards/chts_wfa_boys_p/en/ Perhaps to address the weight loss and eating, the food he's consuming should be more nutrient-rich. He can eat small amounts of food, granted that each amount is filled with nutrients. Offer him fruits over sweets, vegetables, lean meats, dairy products, whole grains, nuts. Here's a good guide on what and how much your child should be consuming regularly, according to the HPB: http://www.hpb.gov.sg/HOPPortal/health-article/6086