I read about half of the posts so far and it doesn't seem like you are getting much of an answer.
Deworming medications are (or should be) prescriptions in your area. They can be toxic, however side effects are zero to minimal in a HEALTHY animal. Your guy has not eaten, and from what I gather, has not defecated much since you've gotten him; which may be a sign that it hasn't eaten much for the previous person either. This is reasonable grounds to classify your monitor as UNHEALTHY for whatever reason. In the meantime, do what you can to optimize husbandry, rehydrate and nurish your monitor. Minimize stress, handling, and viewing. If it doesn't come around in the next few days, contact the herp vet that I"m sure you've already looked up, for a visit. He/she will discuss different diagnositics that can be used to assess the overall health of your animal, and the affects of any treatments that may be necessary.
If your monitor comes around with the above advice and a vet visit is not needed, than perhaps bring in a stool sample to see if deworming is necessary.
Ian