"Is there any truth to what others have told me to “file first” so that means the burden of proof lies on him rather than me?" The answer is No.
If you e-file your tax return and it is accepted by the IRS, then any other party that tries to e-file a return with the same personal exemption as was entered on your tax return will have their e-filed return rejected by the IRS. The other party will have to print and mail their tax return.
Since the IRS would have received two tax returns with the same personal exemption, they will investigate both parties as has been noted several times in this thread.
