What are conditions my 3yr old son might have is he sweats a lot while he sleeps in cold room light pjs and light blanket half off him?

Dysautonomia: Sweating like that is caused by an overactive sympathetic nervous system. Commonest cause is diet, particularly sugar.

Unknown and benign: As we discussed before i'm glad to see that you are keeping his room on the cool side.Your child may have idiopathic hyperhiderosis, ie excess sweating for no known reason . In any event it's nothing of concer. Sometimes spicy food or hot food may be the cause. Sugary drinks may sometime also be a problem.