Are you weighing him? I have one gecko who eats when she feels like it but is growing like a snail and doesn't like any bugs at all. In fact shes older than all my geckos and is the smallest but shes healthy just slow growing. You may just have a stubborn grower. Is he shedding occasionally? Do you see poo around his tank? If everything checks out and he's healthy you probably don't have much to worry about.