Don't stress...the hard part is over...deep breaths!

Passport w/ visa, COPR and birth cert. would be the most he would need I would think.
The interview is not a grilling type interview, it is just for them to make sure your status has not changed regarding your criminal record, dependents, and marital/relationship status. Our interview took 5 minutes and no, the sponsor was not questioned.
I didn't do a flagpole landing but I'm sure it will be no big deal...there are other accounts of flagpole landings on here, try doing a search.
Sit back and relax! You're on the home stretch!! Congrats!