Leopard Gecko stopped eating?

I have a baby leopard gecko that I've had for about 3 months. Over the past few weeks I've noticed he isn't going to the bathroom much at all, maybe once every 4-6 days sometimes longer. He doesn't seem to be eating anything really either. I have him on reptile carpet and the temperatures are where they should be and he has a under tank heater. He went after a cricket today but gave up quickly (I even took the legs off the cricket). I've offered him crickets (gut loaded and dusted with calcium/vitamins) and mealworms on a daily basis and have also even ordered butterworms and waxworms online to see if he would eat them. He ate a fee and then threw them up (they were small enough). I just don't know what to do. Maybe he was sickly when I bought him now that I think of it, he didn't poo much then either. He doesn't look skinny, but his tail isn't as plump as it was when I got him. Help!