My question is: "How did you get a Google Voice #?" I assume you got it on "your" gmail account.
What is more flexible, and a lot longer lasting, is getting your son his own Gmail account, and his own Google Voice #. A Google Voice # can be set up for any Google gmail account, by giving your cell phone number to get the security code needed to set up a GV #, and then removing it.
Google has this habit of grandfathering in people who have signed up for one of their services. By this I mean, if in the future Google discontinues giving out "FREE" Google numbers, those who already have them will remain free. Just because you get him a Google Gmail account, doesn't mean you have to let him use it now, if this concerns you.