First of all, how exciting to hear how much your ds has grown in his reading! I celebrate that success with you!
It is so fun to be able to watch our kiddos grow...especially in their weak areas!
About using extensions: I think that the most important thing is that your ds has the skills for the guide. Because he is in the extensions range, then yes, you could (and I won't say "should") do the extensions. Personally, I would make that decision based on his ability, and NOT on his age. From the way he is progressing, it sounds like he may be ready for them next year (and yes, he would probably use them every subsequent year)!
Yet, before making any decisions, I would ask myself: Is he able to handle all of the daily work or is he struggling with some area? Is he mature enough academically, especially in his writing and reading, to complete the extensions independently? Eventually, imho, yes, you will want him to use the extensions to be on track, academically, but I would really want to make sure he is successful in the guide before worrying about bumping up the expectations with the extensions. Of course, that is just my opinion.
About using Rev to Rev with a 15yo: I think Carrie has done an excellent job of explaining different things we can do to beef up a program for high school There is a discussin at the top of the board (I have not figured out how to link to it) that contains other links to additional posts about this. I do think you will have to add some things to it, but Carrie explains some ways to make this program high school credit worthy - and thankfully, there will be many who go before us to share their wisdom.
HTH
