How do you help a tree sparrow with a broken wing?

I am an exotic animal vet and work with wildlife from time to time. If you are not a wildlife vet, and I assume you aren't because you are asking the question, then i would take him/her to your vet for humane euthanasia.

Fractures of pet bird wings are traumatic, but fractured wings of wildlife are usually fatal because of the high potential for compromised function even if the bones COULD fuse and heal, and the very high expense involved.

If you have a lot of money to spend on a bird that you will eventually let go (and then maybe he will starve and die from compromised wing function or maybe die after being pounced by a cat) and release to the wild then do it. try it. invest in our wildlife.... but generally, it is too much money and too long of a time, and far too STRESSFUL on the bird for it to be undertaken. I have literally seen wild animals die from the stress of interacting with humans.

Euthanasia is the most humane treatment 99% of the time.

Bearing in mind that wild animals are not humans and do not benefit from human interactions, attempt to keep them in a warm dark and quiet environment until you can get them to your vet. Do NOT pretend that you are a veterinarian and try to set the bone yourself.