By "stick-tail" I'm assuming that you mean the tail has become skinny. This means that he has been losing weight over a long period of time. This usually happens because of a disease process or incorrect care. Since this has been happening for a long time, it's not an emergency, but it's definitely wise to visit the vet tomorrow.
Without knowing what caused this, I cannot recommend anything in the short-term to make him feel better. I can only recommend that you ensure that he is being kept at the optimum temperature and environment. I've given a link below to a website that has great information about proper care of Leopard geckos.
http://www.anapsid.org/leopardgek.htmlI hope this helps and Garbanzo gets better soon.