Hello,
The first thing I would do is look over our caresheet on leopard geckos found here:
http://www.herpcenter.com/showthread.php?t=6982
What you are describing does sound like parasites and he needs to see a vet. Force feeding him works for the very short term, but he won't gain weight and get healthy if the parasites aren't taken care of.
His heating could have also been inadequate before. If it was, the leo could have stopped eating since it couldn't digest its food.
If you will be force feeding the little guy until the vet sees him, I would suggest mashing up waxworms with some calcium/vit. supplements added. The waxworms are high in fat and will be good for him right now.
Set the vet appointment up asap though. The tail is the storage compartment of the leopard gecko. If the tail has no nutrients to draw on, it will die from malnutrition.