I think sons have such a complex way on a mother's heart. I think the open mind aspect is to not rush to the assumption that the lack of a introduction is because of the worst. Not that you the OP don't have reason to have a multitude of emotions about the topic. You mention being close.
Maybe ask him with a hypothetical? If he were to have a gf, how would this play out in introduction? Has he had other gf's before that you've met? And what happened when he did?