You can go ahead and claim your son. You would have to print, sign and mail your return to get it accepted. Since you appear to be the custodial parent based on the number of nights your son lives with you, his mother would not be permitted to claim him unless you provide her with a form 8332 to release the dependency to her.
The IRS will contact both of you later in the year requesting information to support the claim that you are the one who can rightfully claim the child.
