I agree that it's not just because of your partner - I think some guys are more attracted to boy moms, others are more attracted to girl moms and so one guy can have the same gender with more than one lady and it doesn't necessarly mean that he's shooting more Y than X (and that's not even possible.)
That's not the way that stress works, rather than giving you a full explanation I need to actually write an essay on that but short version, stress can raise or lower testosterone depending on a lot of different things. Chronic, longterm stress actually lowers testosterone; it's actually not the stress he's under per se that would raise his T levels, it's the fact that he's in charge and has to be in control of a lot of different things simultaneouly, always meeting and overcoming challenges, that would be upping his T levels.
Higher testosterone can mean higher sperm count and more boys BUT that DOESN'T mean you can't get a girl. There are tons of ways to lower sperm count.
I have seen a trend where chefs seem to have more daughters so he may be "due" for one anyway LOL. Please don't despair, if you look at Gordon Ramsey he is constantly stressed and hollering but still has three daughters.