My rat is sick and i don't know whats wrong.?

I have a 5 month old rat and hes shaking and wont eat or drink.This started yesterday and hes lost some weight.hes had food but it doesnt seem like hes eaten any and he seldom drinks water.hes not energetic like he used to be and his eyes are half open and his body temp is cold.idk what to do and i dont want to lose him.what should i do?