Yeah, y not, just for courtesy sake i guess it's ok to inform your Boss. I'm sure on compassionate grounds, he would allow and respect your privacy. No harm in asking or informing.
When i was working then, i expressed my breast milk within my cubicle space. A friend seeked permission from her BOSS to return home just to express her milk. Lucky her, coz her apt was just a few blocks away from her workplace.
all the best!