~.~.
AP classes do add a point to your GPA. So in a regular class, an A would be 4 points, a B 3 points, etc. An AP class means an A will be 5 points, a B 4 points, etc. The grades will remain the same, but the GPA weight is higher. I graduated with a 4.13 GPA because of the AP classes I took. Never hurts to have a high GPA.
Also with AP classes, there is a test you can take and possibly get college credit for the high school class. The tests are scored 1-5 and usually a 4 or 5 will get you the college credit. Sometimes colleges will take a 3. You have to pay for the test, but I was fortunate that our school district paid for ours.
Colleges are going to look at honor courses, pre-AP, AP, etc. So, I would encourage your son to try to take as many of those as he can, assuming he can handle it. I don't agree that a C/D in an AP class is better than a 99 in a basic course. If your son can get at least a solid B/C or higher, then go for the honor/AP courses.
Sorry, I don't know anything about AVID, although it sounds like it could help him.