Forgot Plus One......Can I add one now?

My cousin just recently started dating someone and I wanted to be fair and include everyone's significant others on their invitations but I didn't think to invite her because everyone else's plus ones are people that they've been with for over a year. Both his brothers' significant others were included but I honestly didn't even think to include his girlfriend because his mom (my aunt) told me that she wouldn't be invited to his own brother's wedding (he recently got married) because it was last minute and the relationship is so new. But he's the only person now in his family that hasn't RSVP'd and I'm afraid maybe he feels hurt that I didn't include a guest on his invitation. Should I tell him it was an honest mistake and that his girlfriend would be more than welcome at my wedding? Or is it too late? It's not really an issue of budget, I just want to make sure I don't hurt anyone's feelings.