car seat question

Hannah • Hey my name is Hannah. I am a 27 yo SAHM 💙👶🏻 💙4/29/16 💙👶🏻 💙4/23/18
I'm a FTM and my son was born in April and hasn't needed a jacket yet. As it gets colder I'm concerned about the jacket issue. What are somethings I should know? Is there anything I could buy for his car seat to keep him warm? Should I also avoid putting him in sweaters? TIA