Yes, he will have to submit his original Russian-issued BC, stating he was born in Russia, with his I-485, if he does not have a Ukrainian-issed BC at the time he submits I-485.
If he attaches a BC that says he was born in Russia and puts on the application that he was born in the Ukraine, he'll likely get an RFE.
They will not send him request for RFE, because his eligibility for Ukraine could be based on his parent's country of birth or because he could have been born in a part of Russia that now belongs to Ukraine (and at that time it belonged to Russia; BTW there exist some lands transferred from Russia to Ukraine under the Soviet Union, for instance, Crimea; a lot of people were born in Crimea). That is a responcibility of interviewing CIS officer to figure out what the situation is.
In case he was born in Crimea when it belonged to Russia, he had and has to write on the forms he was born in Ukraine (because it is Ukraine now), and he had to provide his Russian-issued BC stating he was born in Russia (because at the time of registration of his birth and issuing his BC it was Russia). The instructions for I-485 do not require any kind of explanation about this kind of situation. That is a common situation and should not generate RFE, because everything was done according to the instructions from the very beginning. RFE is issued when some instructions were not followed correctly.
The fact that his BC changes by the time of interview is not an
additional problem, because the Ukrainian-issued BC will have the date of registration of birth anyway, and that date is in fiscal 2008 anyway. Regardless of any other circumstances, that fact will be clear to interviewing officer at the time of interview. And together with his hospital papers and affidavit from his mother, and with the story he told us, that should not be a problem.
I mean even if he submits Ukrainian-issued BC with his I-485 from the very beginning, the situation is exactly the same, because the BC still carries the same registration date. So, the officer will understand his birth in Ukraine was not registered until fiscal 2008.
So, your argument that changing BC during the process is worse than submitting Ukrainian-issued BC from the very beginning, does not work.