What is a good cheap cell phone plan for my teenage son?

Well, I have two teenaged sons, and while the one isn't a cell phone user, the older one never goes anywhere without it.
The main thing I've learned with him is this: If the phone company offers you an unlimited texting plan, take it! We used to have a plan in which he and I shared 1000 text messages between the two of us, and it worked really well until he met a girl who loved texting...and my phone bill in one month jumped by $150 for the overages.
The best thing to do is if you already have a cell phone, ask your phone company if they can add a second line to your plan for an extra amount. Most companies let you add a second line for about $10, and if the plan you already have leaves you with several minutes left over at the end of the month, you and your son can share the minutes. Then add an unlimited texting/Internet plan with it, and you should have a good deal. Those should run you $15-$20 a month.
However, here's what I've been looking at. Boost Mobile is now offering a $50 per month plan for unlimited talk, texting and everything. You do have to purchase a compatible phone, and since they're not on a contract, they can be a little pricey. However, for $50 a month and you don't have to worry about overages or a credit check? That's pretty good.
Cricket also has an unlimited plan for only $45, that will give your son talk, mobile web, texting and many other features that are unlimited and again, with no credit checks or contracts. They are even having a cell phone sale, with very inexpensive telephones.
After looking at these, I am seriously reconsidering not only his current cell phone plan, but my own!