Ohh boy.
Awwz...no worries, I'm pretty sure he was just kidding
In any case, you do have to stand a lot on MS3 in some rotations, I guess the most troublesome would be Surgery and OB/Gyn. Medicine may also be problematic b/c of loooong rounds where you're on your feet the whole time. Do you see a doctor for your back problems? It might be worth talking to someone (doctor, chiropracter, whatever) and see if you can optimize things before you start the year. Maybe they can give you some heavy duty NSAIDS or maybe a brace you can wear or something.
In terms of minimizing time spent on your feet, someone already said avoid long surgeries, which is probably a no-brainer. I'm on Surgical Oncology right now and am probably on my feet and scrubbed in for 8-9 hours at a day with surgeries lasting anywhere from 3 to 7 hours, and you have to stand the whole time scrubbed in. On most general surgery rotations the majority of cases are pretty short (Under 2-3 hours). If you really do get into a pickle, the attending, resident, or scrub tech will let you scrub out to sit down for a bit, but your concern about looking lazy is valid; I would not ask to do this more than 2 or 3 times.
For OB, try to get deliveries mainly on multiparous women, the more kids they've had the faster it goes. If you get a prima gravida, you may be stuck standing there telling her to push for up to 3 hours. If it's someone's 6th kid, it will likely be over in 40 minutes or so.
On Medicine, you may get lucky enough to have an attending who does sit down rounds, although the vast majority have you walking/standing the whole time (usually 3-4 hours). One trick you could do is when they aren't discussing your patient or are moving between rooms, dart off and check lab results on your pts that might have come online in the last few hours. Your intern will love you for this, and you can sit down AND be helpful at the same time!
For the others like Psych, Neuro, Family, and Pedi I found that there was usually time enough to sit that my feet/back did not ever bother me (but I didn't have back problems coming in).
Best of luck, I'm sure that you will find a way to get through it, hopefully my suggestions help a bit.