First off, congrats!!
Initially, the recommendation below is a good one--it will help keep your leather from getting scratched/imprinted. I had my son last summer and we didn't have to worry about any spills really until he was close to a year. I'd say one helpful hint is to have your wife/significant other ride in the back if the baby needs to be fed, so they can prevent any spills. As they get older though, there will be a period where the "no food in the car" rule might be null and void--when a toddler is hungry you feed them and you might not be able to stop.
Regularly inspect/clean the backseat as necessary and get a good leather protectant. As they get older, teach him the importance of being careful in the car, that the car isn't his personal storage bin or trash can, and to always clean up after himself--every time he gets into/out of the car. I have a 10 year old stepdaughter I'm teaching that to currently.
:iagree: