I did a move when my little girl was 3 months old and it wasn’t as long but still from one end of the uk to the other. I just mapped out stops and we took our time. Lots of stops and just as organised as possible with bottles if you use them, toys and any comforts they need
I'm sure I was told that babies can't be in car seats for more than 2 hours at a time. It's to do with their spine and changes when they can lift their head up. So do frequent stops and get your son out even if it's for a 5 minute stretch