Could you describe what you are seeing in more detail? I'm not quite getting a picture of "not using the front legs".

Is he eating in his enclosure? Signs of impaction are usually straining to defecate and regurgitation of food.

A more common problem with young growing lizards is calcium deficiency. Their diet is not in proper Ca (calciumhosphorous) proportion because they eat a lot of pinkie mice, insects, eggs, and ground turkey. Perhaps he needs more supplementation with Vit D or a different arrangement with the UV bulb. Proper temps are essential for metabolism of these nutrients as well.

Just some thoughts....