Is my SO lying?

So my SO said he doesn't want to introduce me to his parents as his girlfriend because his mom wouldn't like that he's dating a white girl, so he wants to wait until he finds a good way to tell her. I just find that hard to believe because my SO is white. His dad is white and even though he says his mom is full black she doesn't look it. I'm pretty sure she is half white. His mom is also super nice, I've met her before as a friend but he said it will be different as a girlfriend and she will be disappointed. Do you think maybe his mom just doing like me or is this really about race?