Heather,
Yes, you will definitely want a Convertible Car Seat (one that fits both rear-facing, and forward-facing) since your big little guy is only 6 months old. He still needs to be rear-facing until at least 1 yr old - and ideally for longer. (not to scare you but the main issue with turning a small child forward facing when their head is still 30% of their body weight is the risk of internal decapitation or other serious neck/spinal injuries in even low speed impacts)
So a convertible seat would be the way to go!
Recommendations: I would highly recommend the First Years True-Fit http://www.elitecarseats.com/the-First-Years-True-Fit.pro
It will keep your son rear facing until 35lbs, and then forward facing in the 5-pt harness (safest way to travel for children!) until 65lbs. By time he hits 65lbs you will prob. only need to get a small belt positioning booster at that (years down the road from now). the other thing I like about this seat is it's removable head extension, the quality and fit, and the safety ratings.
Also it is less expensive then a Britex wiothout sacrificing safety.
Other seats that have a high weight limit and will work for the long-haul are:
Radian XT by sunshine kids http://www.elitecarseats.com/Sunshine-Kids-Radian-XT.pro#... rear faces up to 40lbs, then forward faces to 80lbs in the 5pt harness. extra side impact head protection, and slim design means you can fit 3 of these seats across the back of a a small sedan.
Britax Marathon http://www.elitecarseats.com/Britax-Marathon-CS.pro
the standard in carseat safety. known for thier rock solid installations (no budging) and very plushy and comfortable.
I'm a big fan of convertibles that will last for YEARS and not be outgrown for height before they are out grown for weight. these 3 fit the bill, and considering they are expected to save your little ones life - they are a great investment.
Small tip: remember to save your receipt! If you are ever in an accident, the insurance company will have to reimburse you the full amount of your carseat(s). This happened to us last year, and only my Amazon.com recipets were able to get me the full amount back for replacement seats.
good Luck!