E.B.
Hi M.,
I hate to tell you this, but if you want him to sleep well at night - he MUST sleep during the day and it is the catnap in the car that is robbing him of his good afternoon nap.
You might want to consider at least one day of the weekend working your schedule around his nap. Your baby has a routine during the week and is probably confused on the weekend without it. He doesn't know when to sleep or eat or play. Feeding him on a fairly regular schedule also helps him take his naps and sleep at night.
I have read LOTS of books on this because my baby refused to sleep at 3.5mos...he is now 10mos and sleeps much better. I recommend you read The Baby Whisperer by Tracy Hogg - I do not take all her advice as gospel, but it gave me a new perspective on how my baby sees things.
Hope this helps.
Liz B