My gecko is really thin and possibly sick?

Hi there, this is my first post here. Excuse me if I do something wrong lol. Well, I've had my gecko Aristotle for over a half year. The place I got him at had him solely on crickets, and the guy told me that Repashy should NOT be used as the main source if food. (which I know is not true) I had to hand feed him for a while until he could eat on his own, which I'm sure he has, I've seen lick marks. He looks really thin and I'm not sure what to do, I've had very many pets before, but not a reptile. I'm not sure how to post pics yet, any help would be appreciated. I like to give him crickets every now and then, if I can. Please help, I'm really worried. Sorry for the long post, I'm just worried for him.