In comics, Star Boy's name is Thom Kallor (not Thomas) but in Justice League vs. the Fatal Five, his name was changed to Thomas Kallor. But why was this change done? Is it something to do with Bruce Wayne's father's name which is Thomas Wayne? Or some other reason for the change? As changing name to short form is not so unusual but enlarging name might have some reason behind it.
Tom and Thom are shortened forms of Thomas in the same way that Will is a shortened form of William etc. If someone is called Tom/Thom, it is perfectly plausible that their birth certificate will say Thomas (and possibly they might be called that in more formal settings/by their parents etc).
Thom is also a first name variant of the abbreviation "Tom" of "Thomas" that holds the "h".