How old of a kid? My understanding is that IPv6 is for when the normal IP range runs out. I can't think of anything nefarious anyone could do with this.

As far as having an interest...why not? I have a friend that tells me about injection attacks and buffer overflows and all kinds of crazy things I never worry about. He was into this stuff when we were in college. He's now one of the highest rated Ciscso security people in the country.

I don't suspect anything nefarious as he is a high school dropout and doesn't know how to do anything beyond basic html. He doesn't know what SSL means. He said he has done some downloads using it.

My concern is that he may be violating Comcast's terms of service by invoking IPv6 unnecessarily. Getting service cut and jumping through hoops to get it turned back on would be highly inconvenient.

So I'm coming to this late but...

There are some transitional technologies that will enable you to tunnel IPv6 in IPv4 so you could, perhaps, use that a bit like a VPN (although I doubt that it would be secure at all). If that was the case though, it would just behave like IPv4 until it gets to a gateway (which would behave much like a proxy, therefore disguising the destination of traffic from the client).

Alternatively he may see some advantage to having a publicly reachable IP address within his network without needing to do stuff with NAT. Not sure what that would be...

Of course IPv6 is so little used it's also like a kind of hidden internet - and which kid could possibly avoid the temptation of that (I'd be tempted myself but I can remember what an empty internet looked like the first time around).

Anyway, I have no idea but I'm glad someone else gives a shit about IPv6.

IPv6 is just an extended Internet addressing protocol. It's a sort of equivalent of adding "USA" (or whatever country you are in) to the last line of your address on letters. IPv6 actually went into use big time earlier this year, as the Internet is running out of IPv4 addresses.
Comcast is already using IPv6. He is just playing - this is really unlikely to contravene any ToS agreement. It's like typing in the numerical IP address instead of the URL. It's really quite fun to play with this, to discover what your IP address tells the world about you (the link here is for IPv4 but you get the idea).