Interesting, I've just found the phrase "prelim boy" in The Mating Season by p.g.wodehouse. In the context of comparing a domineering aunt to wallace beery, who played a boxer, I assume a prelim boy is someone on the preliminary bouts of a fight card.
I think you may be right. The full term is also used in the 1937 novel
There Ain't No Justice, by James Curtis, which is about boxing.
BTW, welcome to the forum. You should add your type of English and location to your profile (see under my avatar for an example). That information is often important here.