**Use thin, warm layers under the car-seat harness, and add the bulky warmth over it.** Your clothes-and-blankets approach was sensible, provided the blankets went over the buckled harness.

A thick snowsuit can compress during a crash, leaving the harness dangerously loose—even if you managed to buckle it. A bigger seat doesn’t solve that problem. [American Academy of Pediatrics guidance](https://www.healthychildren.org/English/safety-prevention/on-the-go/Pages/Winter-Car-Seat-Safety-Tips.aspx)

For everyday trips:

- **If you carry him in a removable infant seat:** buckle him indoors wearing snug layers, then add a blanket over the harness. Carry the seat to the car and into the building with the blanket in place and his face uncovered. Use this approach only while he remains within the seat’s size limits.
- **If the seat stays in the car:** carry him outside wrapped in a blanket or wearing his coat. Remove the bulky layer before buckling, then place it over the secured harness. At your destination, unbuckle him and wrap him up or put his coat on before carrying him inside.
- **For a longer walk:** put on his outdoor coat or snowsuit after unbuckling. Allow more protection for wind, wet weather and extreme cold than for a quick doorway-to-car transfer.

A bodysuit, trousers, sweater or thin fleece, plus a hat, mittens and warm socks or booties, makes a practical starting outfit. Young children generally need about one more layer than an adult in the same conditions. [AAP cold-weather guidance](https://www.healthychildren.org/English/safety-prevention/at-play/Pages/Cold-Weather-Safety.aspx)

You don’t need a special snowsuit-and-seat combination. Avoid padded inserts or bunting that goes beneath him or between him and the harness; follow the seat manufacturer’s accessory instructions. Remove extra blankets as the car warms up.

You’re right to keep him warm—the useful adjustment is making the warm outer layer easy to add for the walk and remove for buckling.