Please view the sticky though. I have heard from some people that were surprised when the GAC did not get get them to the front of the lines. Not saying that was your intention at all. That sticky has some great information in it.
Have fun!
In fact, the lines that DO have an alternate waiting area (not all do), often will have a longer line than the regular standby line. Also, sometimes the alternate waiting area gets just as crowded as the standby line.
GACs are not designed to shorten wait times, so plan on waiting at least as long as the standby time when you go. For example, last time at Jungle Cruise, I ended up waiting 45 minutes instead of the 10 minute standby time because there were a lot of people with wheelchairs (and if you use the stroller, it will count as a wheelchair in this case) in front of me.
A lot of rides have a limit as to how many wheelchair users can be on at a time. CMs don't really care how well you walk, if you show up in a wheelchair, it counts. Most Fantasyland rides, that number will be 1. This means if there are 3 people with wheelchairs in front of you, you have to wait for the ride to cycle 4 times before you get on.
You are going at a VERY popular time, so do the best you can with fastpasses, and realize that you will likely not get everything you want done. A GAC will help, but a touring plan may help more.
GACs will not work for buses, restaurants, stores, character lines, parades, street shows, or fireworks. When he is on the bus, you will have to fold the stroller and have him sit in a regular seat, just like any stroller.
Hope some of this info helps!