Dogs big and small love to chew. It's in their nature. If yours loves to chew on your underwear, though, you need to either correct the problem or invest in seriously durable boxers. Fortunately, keeping your undies away from a little dog is easier than keeping them from bigger pooches.
Take away any underwear you catch him munching on and give him a stern "No" command. Replace the underwear with another chew toy, and when he uses it, praise him.
Keep your underwear out of reach. The best offense here is a good defense, and since you're dealing with a little dog, it isn't hard to put things where he can't get them. Keep dirty underwear off the floor and in a hamper, or use a baby gate to keep your pint-sized pooch out of any room where dirty undies live.
Give your dog chew toys of varying textures. For example, give him something hard to exercise and clean his teeth and gums. And give him a soft toy that is gentler on his mouth and mimics the feel of clothing. Don't indulge him by giving him old and tattered underwear, though. This represents a choking hazard and sends him a message quite the opposite from what you want.
Exercise and play with your dog. If he's a compulsive chewer, he probably has pent-up energy that he needs to burn off. Take him for a walk or play with him, but remember -- he's just a little guy. While he needs regular exercise and stimulation, you can't quite put him through the wringer like bigger, more muscular dogs.