I made my boyfriend cry, now what ?

my boyfriend is very sad because I asked him what he was doing for his mom on mother's day. He's always admired my mom, and calls him his mom, but I never thought much of it. Turns out I've known the man for four years, and I never noticed the absence of his mother.... His was response was that he wishes he could do something for her, but he can't, she's not here. ... I immediately began apologizing, and he said it was ok. He then left and has been sulking ever since. :( I honestly didnt know that his mother passed. I feel horrible, and really sad now. The thought that he is in pain, makes me cry. I don't want to bring it up again. But I don't know what to do either ... just give him space? comfort him? any advice, I've never actually dealt with death or grief in anyway or form other than a pet... :(ughhI just feel so thoughtless, selfish, and uncaring now.