I was thinking this too. I'm generally a big fan of the Oxford comma.

Just an fyi, Oxford comma only applies to lists of 3 or more. I think this is just a dangling modifier.

This is a list of three: Whitson, his son, and his girlfriend.

Girlfriend of the whitson or the his son?

Beats me!

The Oxford comma isn't the issue here. The issue is that "his" binds both son and girlfriend, and therefore needs an "and" in front of it. It's not grammatical as is.

It would still read perplexingly. The Oxford comma isn't a tool for the inept; rather, the ept.

It's interesting how sometimes what appears to be a prefix, isn't.

That is an affront up with which I will not put.