Simple, fitted clothes work best. You don't want them too tight, but clothes that are comfortably snug are best for photographing babies. Avoid collars, or overall straps that rise around his face when he's sitting. Avoid patterned fabric. Pick one nuetral top that will reflect well on his skin. For a colored top, go with whatever reflects the most intensely in his eyes.
If you want to help the photographer by getting your baby to do his best smile, be sure to listen to where the photographer needs you. If you stand directly behind the photographer, they are more likely to get the babies eyes right in the lens. To the side, you can get a cute partial profile. If you crouch lower than the camera, your baby may follow you with his eyes, and the photo may not engage the viewer the way it should.
You shuld be prepared to step in for a few photos, even if its just your arms and hands. Your son may decide that big smiles happen only in mommies arm that morning!
Good luck! I'm sure it will be so fun.