Hi MC,
Getting our kids cell phones is such a big step isn't it?
It really depends on your son, and how he will use his new phone. I was just at AT&T a couple of days ago, and the lady said the biggest "misconception" about the "go-phones" are the texting, and im-proper use of the service agreement.
I think anyway will work for you, as long as you're aware of EVERYTHING (web broswers, texting, photos, minutes, etc)
I just got my 12 yr old his first phone a couple of months ago. He is on our "Family Plan". Although my son is a little older than yours, your son will still be very excited to have a new cell phone. I sat down with my son and went over his phone rules. For example, 5 texts allowed per day INCLUDING incoming, he will ALWAYS answer my calls, always EXIT the web browser, nobody is able to use his phone unless it's for an emergency, etc.....
He knows what is expected of him, and my son is doing a great job on his phone rules................so far :O) Middle School starts in a couple of weeks, so we'll see about it then.
In our case, I think it's important for my son to have a phone. He will be at a different school than I work at, and his brother goes to. I will also be a busier "Taxi Service" this year. As the oldest, there will be times he is waiting for me (for once in his life).....I feel better that he will have a phone to always contact me.
In your case, 9yrs old is still young, but you seem to have your reasons for considering this responsibility. Just make sure you are not getting him a cell phone because it's "the cool thing to do". Make sure it's when YOU think it's time. My son hounded me for a phone since 3rd grade, but just recently got his. My decision was based on when "I" thought it was time, not when "HE" thought it was time. My son respects his phone more because of it (again....so far).
I'm sure whatever you decide will be just fine for him.
~N. :O)