If my son worked 1-2 months in 2018 how do I file his W-2 with mine?

You don't add your sons W2 to your return.  If he is required to file a return, he would file his own return.  If would need to check that someone is claiming him on their return. If you are using the Online version, you would need to create a new account for his return.  You should be able to use the Free Edition for him.