Need to know who claim my child

Why?

This has become a common issue since electronic filing. If someone else files claiming your child, then you are blocked from filing electronically. The IRS is not going tell you who it was.

The solution is to file by mail. The IRS will then review the returns and take appropriate action.

The IRS doesn't have to tell you.
You claim him if you have the right to (more than half the nights of the year at your house) and the IRS lets you know if someone else is claiming the same, and penalizes the one who is wrong.
Why would it be anyone other than the other parent? How many people are involved in taking care of him?

You will not know or find out. If you have the right to claim, you will need to file paper filing and note and show you have the right to claim.