No - The date he used for his signature should not affect your return unless he did not file an additional extension beyond the ordinary 6 month extension period (which for a Form 1040 is Oct 15th).
If the return is dated Dec 17, 2017 then that will be considered the date related to the return. The fact that the return was filed more than 2 months after the original extension date would more likely be the reason for the delay.
Also find out if it was mailed certified or if he has any record of the filing.
