Did you get a weight on him as has been suggested? Thinking he might be losing weight and knowing he is losing weight are very different.
If you have not gotten a gram scale yet, run-don't walk- to your nearest office supply store, etc., and get a scale.
Record keeping is a huge part of husbandry responsibility. Your records will allow you to track your herp's progress while also giving the vet a really good picture of what's been going on (should he ever require a trip to the vet.)
In our journals, we have spaces for date of feeding, what fed, weight of prey, and weight of herp (2x month when they're young.) Also have a space for when shed, success of shed, and a 'notes' section.
Good luck; and get the scale!!! :D