S.A.
Some informative websites to check out:
http://www.usa.safekids.org/skbu/cps/infants.html
http://www.seatcheck.net/boostillinois.html
Pretty much, they state that your child should remain in the back seat and rear facing until 20lns and 1yr old, and longer if you have an adjustable seat.
Babies are extremely flexible -- I wouldn't worry about your child being uncomfortable. if you are worried about discomfort on a long car trip, then you could stop every hour or couple of hours and let him stretch his legs.