@Independentlizzy - simply claim your son on your tax return and mail it in (you can't efile)
The IRS will figure it out.
I'll assume this is a parental situation for simplicty.
the person who can claim your son is the parent who housed the child for at least 183 nights of 2021. by 'housed' I mean the child slept at that person's house overnight. Temporary absenses (i.e. went on vacation with parent and stayed at a relatives or hotel) would still be counted as part of the 183 nights
Be sure you have your documentation. For example, able to show school transcripts from the local school would prove the child lived with you, esp. if the other parents lives far away,
