Just say it straightforward. Duh. "I'm pregnant now and I don't know who the father is" It think that solves a lot of things/misunderstandings.
The sad part is that from what I can tell only royalty can impregnate the same sex, so the number of culprits aren't many. In his position saying he's pregnant should immediately alert the person he slept with, and in the case of let's say accidentally sleeping with more than one person (cause he was unconscious or something ) a DNA test would solve everything. But this being yaoi with plot he is going to choose the most stressful route, stay quiet, and angst over who he got dicked down by. ┑( ̄Д  ̄)┍
A young rich guy who wants to be a cool badass delinquent like his mom's friend.