Go to the library and get yourself a book on dog training.

Take him for a walk every day, and work on training then. Cold or not, he needs his exercise, and it will help both of you.

Make it a routine: when you get home rom school, for example, after you've come in, greeted him and the humans, and gotten all your stuff put down, snap on the leash and take him for a walk! It doesn't have to be for miles, just make it a pleasant time every day. When it's really cold, walk faster, it'll keep you warmer, and him, too.

Take him to parks, and make him sit when you get near friendly-looking people. Most people who like dogs will cooperate if you ask them not to pet him until he's sitting - explain you are training him. With really little kids, if he gets extra wiggly (like every Boxer I've ever known, they've al looooooved kids) either make him sit and hold him between your knees, or, once you've trained him to lie down, have him do that.

But tomorrow, go to the library and get and read a dog training book. It's free, so no one can object, right? Hey, and once you've got him trained to walk politely on a leash, maybe you can walk other people's dogs for money, once they see his stellar example!

Call your vet or local Humane Society, and ask about low-cost classes.

There's no "easy fix" but with effeort and consitency, you and he can become a great team, and everyone will be happier for it. Right now he's probably bored and confused. You working with him, and training him will help both those situations!