Our temp is also set around 68-69 during the winter. When mine were babies, during the winter I had them in footie Pjs, and then a sleep sack over it (Halo brand are good). Depends on the child, though. If you notice your son is a little heat-box, you may have to adjust for him. Same for the other way, if you notice he gets cold easily, you may need to layer more with an undershirt under PJs. My kids are kind of total opposites. DS1 was always cold, so I had to dress him pretty warm to keep him toasty enough to stay asleep. DS2 is a little hot box, and to this day, usually flings his blankets on the floor and sleeps on just his sheet. He has one little throw blanket that is his favorite that he allows to tuck him in, but its hit or miss whether he leaves it on. He never wakes up cold during the night, though, so clearly isn't bothering him.
Under 75 is sweater weather? I'm in shorts and a tank top at 75. LOL. amazing how we all view temps so differently.