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.