How often are you handling him? I remember reading in the Barker's book that animals that are handled a lot and are fasting will lose weight far more quickly than those that are handled minimally.

I only had one snake fast for any significant period of time. He fasted for 8 months and lost about 50 grams in that time. I ceased all handling except what was necessary for enclosure maintenance.

If he lost the amout of weight you say yours has, he would have been at the vet last week!