I found mold undder my babys stroller base. How will I know if that effected him?

Unlikely to harm. Mold is basically everywhere. When you see the allergy report on local weather, there is hardly a day without mold in the air. A few types could be a problem in damp places that are not well ventilated. Symptoms would be respiratory, probably just a cough if allergic. Just clean the stroller with a bleach solution and don't worry.