of capitalization or punctuation?
OK, now for an answer.
eFile as early as you can. If your ex- managed to file before you, your return will be rejected because your son's SSAN has already been used. If your return is processed first, your ex's return will be rejected for eFile.
Then, file the same return by mail, ie., a paper return. When the IRS worker enters your information, he/she will notice that your son's SSAN has been used and whose ever return will be on-hold.
The IRS will send both you and your ex- letters, telling you that two people had claimed your son as a dependent, and will ask each of you to show:
Where the kid lives.
How long the kid stayed with you.
You might need school records, medical records, etc.
And, this won't be resolved until summertime.