Bobby is his stage name. His real name is Sam .
There is a rule in Hollywood that no two performers registered with the Screen Actors Guild can have the same exact name. This is to prevent confusion when royalty checks are made up and mailed out. That's why Michael Keaton goes by that name instead of his real name, Michael Douglas. There was already a successful actor named Michael Douglas registered with SAG. Sometimes an aspiring actor's agent or manager will suggest a more appropriate stage name, for example:
Handsome, muscular Floyd Soobner walks into a casting office and hands his resume to the manager. Manager looks him over and says "Kid, you've got a great look, you could make it big in action pictures, be the next Rambo. But not with a name like 'Floyd Soobner.' How about you call yourself something more masculine, more macho.......let's see.....how about 'Slab Bulkhead'? 'Smoke Manmuscle'? 'Rip Steakface'? I'll make a list, you think it over."