We, too, have a son from Russia, and to simplify matters we went ahead and got the passport. Our cruise isn't til Oct but we like to travel and thought it would be just as easy. However getting a passport isn't a simple matter, you have to present your adoption decree, current birth certificate (certificate of foreign birth) , green card and Russian passport. These all have to go to the passport office and will be mailed back to you along with your son's US passport. And be aware that both parents have to be present when applying for a passport for a minor child. There is a way around that with a sworn, notorized document, but we found it just as easy to both appear. Our passport took about 30 days, so you may still have time, plus there may be ways to expedite it, I'd ask at your local office. Otherwise I have no idea if you would have any trouble with having a ctf of foreign birth only, but I'd suspect you might. Hope this helps, feel free to email me if I can help further!
Heidi