Honestly, I think you should approach him about it, awkward as it may seem. If you really see yourself having a future with this fella, it's an important subject to breech.

Now, how to do it? I suppose the best way to go about it is to be browsing 'round his computer with him and stumble upon it again.

He, honestly, seems like a sweet guy from description. I'm sure he can justify why he has those photos.