Snowsuits, coats and car seats


When my son was first born, we were told not to put a thick coat or snowsuit on him while he is in the car seat because the car seat straps wouldn't be tight enough to keep him secure. When can he start wearing a coat? Should I even buy a snowsuit? I hate not knowing if he's too warm or too cold! He's 9 1/2 months and we live in Ohio. Are you buying coats or snowsuits?