DD1 had exactly the same symptoms as you described, and was also tiny with no energy. She was diagnosed with coeliac disease at 2.10, after a blood test and an endoscopy/biopsy. Within 2 weeks of going gluten free she was a changed child.
My DTs had a blood test straight after DD1's diagnosis, but they came back clear, however about 18 months ago DT1 started showing similar symptoms so they were tested again and were both positive. For them, the doctors agreed that, given the family history, they didn't need the biopsy.
From my limited reading, I believe that undiagnosed coeliac disease can lead to lactose intolerance, so there could be a link there, but I am not an expert in this area.
Just to reassure you, going gluten free is very easy now - if your DS is diagnosed, I would highly recommend joining Coeliac UK. They send an annual directory detailing all gluten free foods, both branded and supermarket own brand which is worth the membership fee on its own!!