Formal:
* Anak sa ibang asawa - This literally translates to "child from another spouse" and is the most formal way to say "step son."
* Anak ng asawa - This translates to "child of the spouse" and is also a formal way to say "step son."
Informal:
* Anak ng ibang lalaki/babaeng pinakasalan ng magulang - This translates to "child of another man/woman that your parent married" and is a more informal way to say "step son."
You can also use anak-tuluyan which means "adopted child" but it's not exactly the same as step son.
It's important to note that Tagalog has a lot of nuance and the best translation will depend on the specific situation.
For example, if you're talking to someone who is older than you, you would use a more formal term. If you're talking to a friend or family member, you could use a more informal term.
It's always a good idea to check with a native Tagalog speaker to make sure you're using the most appropriate term.