One way tickets to Europe tend to be expensive (think B fare), and thus the refare associated with making it a one-way is likely to leave little or no money for a credit. The best you could probably do is find another date (within a year of original purchase) to reschedule the CDG-PHX leg (you may be able to change cities). If you don't have a reason to do that, it is likely that your best course of action is to simply not fly the return. It won't be a problem if your son doesn't make a habit of it.