It's a pity those countries don't track stats that well. From what I could find off globaldatalab.org the numbers in Ukraine and Belarus are pretty much in line with the west at around 2 years. Greece, Serbia, Montenegro have highrt numbers at 3 to 4 years. I couldn't find much on Russia.All I know is that he'd likely be more realistic since there are probably a high percentage of significant age gaps in those countries, though they may only be 20%. That may or may not help an outsider, though, even if he is Orthodox and is seeking an Orthodox woman for marriage, for presumably the right reasons.
Just for comparison the Spanish stats included percentage by age difference, for example roughly 7% had an age difference 10+ years.
That said, it's just the mean age and I'm pretty sure there will be enough to go round for the small amount of westerners that will actually go all in and move to those countries in search of a younger wifey.