Interestingly, my Oxford Concise gives the definition of prostitute as:
1) a woman who engages in sexual intercourse for money
2) a man who engages in such activity
3) a person who offers his talent for unworthy purposes
so I guess that your catamite might graduate to become a prostitute and that we just tack 'male' on the front to differentiate.
Over here the term used for a male prostitute pre-graduation would be 'rent boy'.