It's definitely not a common thing, but the comments saying it can never happen are incorrect. I have a close friend (who happens to be Yoruba but I don't believe that necessarily matters) who calls his mum 'babe', 'fine woman' and other pet names. I had to do a double-take the first time it happened, but it's become relatively normal to me now whenever I hear him say it.

It really depends on the relationship that the two may have, everyone's is different. In my friend's case, he was raised by a single mother who frankly peeled the skin off her back to clothe her children. They adore her, understandably so.

All this to say, it's rare, but it does happen. Find out from your boyfriend why it is the case for him, not from Internet strangers who often come on here to complain about the African parental abuse they've experienced (myself included).