I second the comment on :arrow::arrow: VARIETY, I have had many geckos over the past 15 years and I can say the best way to get a gecko to it's optimum weight and size is to provide variety in their diet, and feeding them every day (if they take it), I always try to provide as many different types of feeder as I can, from mealworms to grasshoppers, crickets, dubia roaches, earth roaches, waxworms, superworms, locusts... I use everything and I truly, truly believe that is one of the most important keys to a happy healthy gecko, and, also, during the first year I make my best effort in feeding them every day (every now and then I get too busy one day and don't feed them, but it is never less that 6 times a week), providing calcium, vitamins, right temperatures, and a varied, abundant diet, he'll get to his mature size soon enough.
This is a case where there is just not one magical feeder insect that will make him grow faster.
Can you post a pic?