The AP Stylebook holds that you should abbreviate “junior” and “senior” as “Jr.” and “Sr.” only with full names of persons or animals. Do not precede by a comma. For example,
- Martin Luther King Jr.
- Martin Luther King, Jr.
The notation II or 2nd may be used if it is the individual’s preference. Note, however, that II and 2nd are not necessarily the equivalent of “junior”—they often are used by a grandson or nephew.
If necessary to distinguish between father and son in second reference, use the “elder Williams” or the “younger Williams.”
See also AP Style Names.