his passports - travelling passport doesn't need to be translated. If checklist requires internal passport, then, you have to do it.
It might be easier for him to go to local translation bureau and have all documents translated and certified by the bureau itself. You don't need to have notarized copies. Then he'll send you the scans of original + translations so you can upload them. I did it for my wife for TRV long time ago and then for PR now. If you wish to make the translation here in Canada of any Russian document I don't know if scans will be accepted - ask your local translator.
For credibility, I'm not sure if it's have to be in the invitation letter or separate one, but since he didn't travel it might be quite difficult. He probably own his apartment and if there is no one to take care for it while he's away - it might be an argument. Any other property like chalet (dacha), a car?
Does he have any occupation? Sorry, can't give you exact answers, you have to improvise.