The primary reason I see for beardies going off food is improper temperatures, namely, being too cold during the day. Their habitat should graduate from ~100F (~38C) under the basking light to ~85F (~29C) on the cold side of the cage. This allows them to move from hot to cold or in between as they need it. They also require UV lighting in addition to the heat, and an old UV light may not be putting out the necessary UV for calcium metabolism. Fluorescent bulbs have a fairly short life span (at least for putting out adequate UV) and should be replaced fairly often. Mercury vapor bulbs last a lot longer.
If he's losing weight in only two weeks, though, I would think that there is something else going on than just lighting/heat. Was his appetite low prior to going completely off of food? Other issues may be a concern, such as impaction (from accidentally ingesting cage substrate) or internal parasites... or perhaps he isn't actually a he and may be egg bound? All things that you should probably find a vet for. If you find a vet that will see your beardie, but doesn't know a lot about them, you might suggest that he consult with another vet who knows more. The joy of phones and the internet - it makes consultations very easy.

At the very least, any vet with a rudimentary knowledge of reptiles should be able to do a basic exam and a fecal check.
You have to be careful with force feeding, one to avoid hurting him by trying to force open his mouth to get food in, and two to avoid stressing him and possibly making the problem worse. I prefer not to force feed unless I have absolutely no other choice.