What's the best way to make sure my baby's getting enough vitamin D?

For babies under 12 months old, I don't see a need to give additional supplements unless as Hui Qun mentioned, the mummy her is deficient is a certain nutrient and is exclusively breastfeeding. Formula milks would also contain Vitamin D so if he's feeding well, you need not worry about it either.

An infant can get vitamin D through exposure to sunlight and through consuming breast milk. As long as the mother is not vitamin D deficient, it is likely that her breast milk will have sufficient amount of vitamin D for the baby. http://kellymom.com/nutrition/vitamins/vitamins/#VitaminD