Whoa we're having similar winters - my year-and-a-half boy weighs about 430g and went off food for ten weeks this year with zero ill effects - he actually seems somewhat refreshed after his fast. He was off from Dec 15th - March 1st. Like yours he's a nervous boy and I do think the temperature drop set him off food, but I definitely don't think you need to worry at all yet - a few grams lost is probably just drinking and urinating. Probably just offer less frequently and let him be, my boy started drop-feeding again (on a smaller feeder) after I left him for 3 weeks without offering. Good luck!