When our son was born my now husband and I were not together, so I gave our son my last name. Now that we are married we want him to have my husband's last name, especially with another child on the way. We do not want them to have two different last names. How do I go about doing this?

You both go to court and you name change done for him. So long as its not to defraud then the courts should approve it and it's a simple matter.