Is my Leopard Gecko eating properly?

My Leopard Gecko Does eat but I'm not sure it's enough and I think his tail is slowly shrinking. I have read lots of blogs etc that state their geckos go into a feeding frenzy when their crickets are introduced and that not eaten crickets should be removed after 10 mins because they will nip him - however, my gecko will ignore his food and it's only every other day or so that a cricket will disappear so i know he IS eating something but I leave the crickets in his viv for a few days before removing them if uneaten. He doesn't seem to touch his mealworms at all. Do you think I'm being over sensitive or should I worry. He seems to get through maybe 2 crickets every 3 days - I have had him nearly 3 weeks and was told he is around a year old (8ish inches long) - Also what should his poop look like? it's wierd colours...?