You shouldn't need to change the site URL in the WordPress config. You should be able to just tell the new site that its URL is
Here's how you would do it:
- Create a new site in your Multisite installation. If you're using subdomains, this would probably default to
mysonsite.mymainsite.com; if you're using subfolders, it'd be
- Go to the new site's dashboard, mouse over the site's name in the admin bar at the top of the page, and select Edit Site.
- Change the Site Address (URL) to
www.mysonsite.com, and save the changes.
Note: You'll need to make sure that your DNS entry for
www.mysonsite.com points to the same IP address as
www.mymainsite.com, and that your web server's config knows that
mysonsite.com go to the same WordPress installation. (In Apache, for example, you might have something like this in your config file:
...but different web servers have different configuration syntaxes, and server config is beyond the scope of this site.)