I know you don't want clothes in the stocking . . . that would be boring for a 6 year old to see anyway, but if you could find a bib or onesie with a funny saying or something Christmas related, that could work. Other than that, I would do a little teething toy or other small toys. I like those little linky toys with the animals hanging from them that you can attach to the handle on your car seat. books are nice too! In our family, everyone gets an orange and a toothbrush in their stockings, so if you have a special tradition for your daughter, then continue it for the baby . . . even if he will not be able to use them for a while. Christmas ornaments, bath toys, a special stuffed animal, wrist rattles, the list can go on and on!
ALSO- try to find something that is red, black and white. My son was 4 weeks old for Christmas and we were able to find a bright Starts brand teether that was a red apple with black and white handle. These colos are GREAT for baby to learn to focus!