Question: Is it safe to put my baby in his car seat with his snowsuit on?
Answer: It is not recommended.
While you want your baby to be warm in the coldest months of winter, it is important for him to be safe in his car seat as well. Puffy winter outerwear will force you to loosen the car seat straps in order to fit your baby in the seat; if you were to be involved in a crash, the loosened straps will not protect your baby. At most, you should be able to slide one finger between your baby's body and his car seat straps.
In addition to the safety issue, a bundled up baby traveling in the car for any length of time will quickly become overheated and very fussy. Contrary to popular belief, babies only need one more layer of clothing than you do.
There are a number of options when it comes to dressing your baby for winter weather, and many are dependent on what growth and development stage your baby is at, so that needs to be considered. Ultimately, to dress your baby for winter time travel in a car seat, put him in his normal clothes and include a hat. Use a car seat cover or a blanket to provide extra warmth.