His Birthday and I’m Not Invited ?

My boyfriends 24th birthdays this week & I haven’t been invited to the dinner his family is doing for him. We’ve been dating for almost a year and I would like to believe we have a pretty serious relationship. I invited him to celebrate my birthday with my family so I’m surprised that nothings been mentioned. He has a good relationship with his family, which is one of the things that attracted me to him in the first place, and I’ve never given his family a reason to dislike me (that I’ve known of). I understand it’s his parents house so I’m not about to invite myself but I do want to know what’s going on. Should I ask? How? Or do I just let this go? Part of me was also thinking of just popping by for a few minutes to say hi but I don’t know if that would be seen as disrespectful.