What is a suitable appetite booster for a 6 year old boy, who is full of energy, but is very reluctant to eat his meals.

Low calorie drinks: If he is on growth curve and without medical or developmental issues, then this will likely pass within a few months. Look at what he does eat/drink for calorie loading: whole milk? Sugared drinks? Junk food/snacks? Poor timing of snacks? Trial of low-fat/non-fat milk / diet/sugar-free drinks / nutritional snacks (good in general), may help him find his appetite at meal time. If not, ask doctor.