I added a line to my family cell phone plan with AT&T (I currently have one of the lowest family plans w/no texting, and it is 700 minutes, which I NEVER use). My thought for the extra phone was for my daughters (10 and 8) to use when they go to friends houses, sleepovers, etc. That has worked so far.
A few years ago, I had a Tracphone. It was just for emergencies, so I needed very little time. Take a look at the website. If it still works the same, you had to make sure that you kept the service active, regardless of whether you had used the minutes you paid for. In other words, you are paying for both minutes and activity. With that said, there was a yearly activation with quite a few minutes for $120 a year (and they always had a special to double the minutes for the same cost). (Watch some of the lower cost per minute deals, because they come with one or two months of activity, so you need to add minutes to gain more activity, which would be more expensive if you are not planning on using the time, but need the activity).
It worked out to, (obviously) $10 a month. Even with that, I never used my minutes. The trick is to make sure that the phone stays "active" so that it can be used when needed. If it goes "inactive" you loose the number and can not place a call until you pay to add more time to the phone (you still have the minutes and there are ways to track it on both the internet and the phone).
You can pick the phones up at Walmart, and other places. Sometimes, (if you are not picky about the phone) you can even get the phones for free after a rebate from CVS or online.
Due to job changes, I went with a family ATT plan, and the added line is $10 a month (the same as Tracphone). We share the same minutes now.
If you don't want to be tied in to a contract, the Tracphone worked well for me. I do know that others have pay as you go, and there are websites that compare the deals.
I know we didn't have the phones when we were kids, but it gives me peace of mind that the girls have the phone with them (only when I decide, as they are still in elementary school). If it makes you feel better, and he can follow school rules with it (and you can afford it), do it.