I am hesitant to advise against a Vet visit. I usually go with the "better safe than sorry" principle. I guess there is a chance that he could have an internal parasite of some sort. Your Vet should be able to test for that.

I would attribute the weight loss to the lack of eating. Addressing why he's not eating would be my first action.

Having a scale is invaluable to a snake keeper. It removes the guess work out of many aspects of animal husbandry. I encourage you to get one at your earliest opportunity.