Excessive sweating of a toddler while at sleep. and cold to touch, starting from his head down to back. what's the real problem?

Sweating at night: Children's heads have a large surface area and they sweat to dissipate heat. If he is an active sleeper and wears a one-piece sleeper, is covered with a blanket, he may be sweating from the heat generated by his moving around. His head would have the most sweat then. If he is completely normal otherwise, it is not a problem. Double check that with your pediatrician next time you see him/her.