Hello Jackie! we haven't had feedback from this "patient" :)
that's right some infections can lower the body temp. instead of raising it, he might have had occasionally a bit of temperature before it drops to hypothermia, it would be compatible. is it always low now or only when he feels sick? what value?
why did they think he was dehydrated? does he have diarrhea? blood pressure? bladder symptoms? is he weak as debilitated with pain and fatigue or more like his muscles won't "work" ?
too early to say which specialist if he hasn't already he should consult his GP, do complete blood count, liver function, ESR, liver enzymes to begin with.
He most def needs blood testing done..... to see his liver, kidney functions, even his Iron levels ..... and just a general (whole body check up) He could of had a viral or bacterial infection.....which again can be picked up in the blood tests... he needs to make sure he drinks loads of water...... or he will dehyrdate again.... Best of Luck....
It may be a virus since viral infections can drop your temp unlike the flu which raises it. He could see an infectious disease specialist. How often does he get this? What time of the day and any other symptoms?