On one hand, he's going to have to command some men, and while he's young, too. Blood would work best in this society, and a connection to the commander of the forces would be best. On the other hand, I don't want him a plausible heir to the commander, because I don't want there to be a plausible heir, in order to give his wife enough room for her machinations.
wrassle, wrassle, wrassle.
At the point I'm thinking of female-line descent -- especially useful in a martial society such as this one -- and possibly a morgantic marriage as well. Not illegitimate, because it might prove necessary for him to be an implausible heir, as in, men who absolutely can't stand what is happening rally behind him.
This is because we have another character -- hmm, a woman -- who expected to inherit. (Female-line can't be too much of a problem, then.) The man in question thought she was illegitimate and went to disinherit her. She murdered him, rendering the entire question moot because a murderer can not inherit from a victim. Still, two questions of illegitimacy would confuse issues, since I don't think they would resonate thematically.