Lower leg is *partly* a central core of titanium BUT this titanium is surrounded by .....bone (the tibia) and the fibula is NOT fixed by titanium rod, it may or may not have had internal fixation at all (some do, some don't). At best the fibula has a plate and some screws on the side of it, not a rod down the center, it's too thin.
So...the fibula is absolutely breakable again as only a small part of it will be plated (and maybe NONE of it, depends on the nature of the fracture) and the tibia has bone surrounding the rod and that bone can be re-broken fairly easily. Prob would be difficult to visually SEE if the tibia fractures again. Only difference is that the leg won't BEND like before as the rod will remain straight (well it might bend under enough force...like getting hit by a truck etc...)
Summary:
1) The fibula can easily re-fracture. (Fibula is quite a thin bone)
2) The tibia can also re-fracture again, but unlikely to be in such a catastrophic manner as central "core" is now metal.
3) His other leg is, exactly as before, "natural" and therefore just as susceptible to similar injury.