Took my five year old to his first game this year, so I can empathize with the challenges that come along with it.
First, as you already have, pick a "lesser" game in case he wants to leave early. I was lucky, as my son wanted to stay until he got to hear Rammer Jammer. He wouldn't leave a minute before.
Second, stay in Hoover. It gives you many more options and is a short trip away from Tuscaloosa anyways.
Third, as said above, I would honestly snag Upper deck tickets on the West side of the stadium. Just make sure they are the lower part of the upper deck because it can be tough on kids getting all the way to the top. This will save you money, and give him a good view of the field and everything going on.
The one thing I would also recommend, which I failed at, is get there early enough to see some of the things around campus. The walk of champions, captains, Denny Chimes, and the Coaches statues are all things I WANTED him to see...but got lost int he shuffle of gameday.