First to answer the question on how long you'll use it:
You'll use it until one of two things happen: 1) he's 20 pounds AND one yr old, or 2) he's too heavy to carry around in it.
If you stop using it before he's 20 pounds and 1 year old, you'll get a convertible seat that can go rear facing. Some of these can be used for babies as young as newborn, but they're not really practical for babies younger than about 6 months, unless you have a big kid. It's just too easy to pop that infant carrier in and out to carry them around when they're tiny! The heat factor isn't totally eliminated with the convertible seat though, but since they're designed more "open" it's at least a little better.
Second: the heat factor. No matter what, kids just heat up in those little carriers. Some do say they have better fabrics, but it's like a little bucket, and you fill it with baby body heat...it's going to get warm down in there. We had the Graco seat too. Ours were both born in the spring, so we had to endure the Texas heat with the infant carrier too. I made sure to dress them as light as possible, and in cotton (they pretty much lived in onesies), and tried not to leave them in there too long once we arrived at our destination. For example, if we were going to the mall, I'd stop at a bench for a minute and take the baby out to let him cool off before putting him back in it and carrying on with my shopping trip.
While you're waiting for the transition to the convertible seat, you might try to wash the seat fabric regularly (couple times a month maybe), and keep the infant carrier in the house instead of leaving it in the car, and make sure your car A/C vents are aimed towards where the baby is sitting (probably don't want them blowing at his face...). I always let the car run a few minutes before getting into it to start cooling it off too. And, in the REALLY bad heat of the summer, left the car running and A/c on until I unloaded the baby (take baby in and let him hang out in the pack n play while I went back and unloaded groceries, etc.) Also, if his seat is in the middle of the back seat, he's less likely to heat up from the sun through the window.
Hope this helps you cope with the Texas heat and make a car seat decision you're comfortable with!