Toddler with down Sydrome Gets cold easily and is weak.?

Two times in the past we have taken our two year old (who has DS) to the doctor because he seems to be very weak and almost like in a lazy state and does not have the energy to do much most of the time. We have had blood work done on both occasions and everything has come back fine. Another issue that has come up is since he has been able to vocalize his thoughts he has let us know that he gets very cold when he really shouldn't be. He starts shivering for things like drinking cold drinks, gets cold right away outside on cool days even if he is dressed warm and so on. I was wondering if these two things could be connected and what else we could have the doctor check for. His iron levels seem to be fine based on the previous tests. Thanks so much. Some people probably think we are paranoid but I would rather be safe.