Try putting your sons details into this bmi calculator for kids:
http://www.betterhealth.vic.gov.au/b...ages/bmi4child

It's a great indicator of whether your son is underweight.

If you are concerned, see a dietician to review diet and weight. We've done this as DS has many allergies and he was 5.5 years, 19 kg and 121cm tall.

It is important to keep tabs on weight gain as it can affect energy levels and overall immunity and health. I hadn't realized DS had stayed the same weight all year without gaining as he had a year of working out what he was intolerant to and learning how to compensate for it in his diet.