DS is mostly weaned, nursing every few days. Does the milk go bad in between nursings? I don't want him to get sick.
As the PPs said, the milk you make that stays in your breasts does not go bad. Is there something about your DS's behaviour while nursing that makes you worried about this?
Depending where you live, and how old your DS is, a great way (in general) to keep him from getting sick during 'flu season (in the northern hemisphere) is to keep nursing. This would probably mean offering more often to see if he will accept.
Hope you both have a happy healthy winter! :)