I am no expert on monitors, but would tell you not to loose time and guess this and that and take him to a herp vet ( assuming there is one in your vicinity )
i'd take him to the vet regardless, something caused him to stop eating and lose weight relatively quickly and if you don't find out what happened and fix it there is a good chance that it happens again, especially if it is a large parasite load causing its' diminished appetite, and they are not being cycled out quickly enough.