Not marbling, or not what I'd usually call marbling, anyways. I've had several juveniles display the same white markings/line along the middle of the plastron, just like yours. It's pretty common. Never spreads to the carapace and usually fades away as they reach adulthood.
Marbling is almost never present in any red foots except Cherryheads. Your's is not a cherry head, and therefore I highly doubt he will ever marble.
To me, that looks like some marbling. As turtlepete said above, sometimes it goes away. With uvb exposure, it so,regimes will darken to an almost indistinguishable black. Now on the other hand, if may be the start of a very rare case of northern marbling, which would be super cool! I don't have a picture of when my cherry started to marble, but it did start something like that. Then little white specks started developing between his schutes on his carapace.