Keep the puppy warm. What may feel warm to us, is not that warm for a puppy. An easy way to do that would be to put a heating pad in part of his crate. (Make sure that you use a low setting with lots of padding between the pup & the actual heating pad. Make sure that there is an area that the puppy can get off of the heating pad all together if it gets too warm.) You can also use a heat lamp or hot water bottle to help out.
Feed meals very frequently. If the puppy misses a meal, then it's not a bad idea to use Karo syrup to help keep sugar levels up. NutriCal would be a good supplement as well. It's high calorie, & will help boost energy levels.