If you want to say anything, I'd just ask if he's had/having much luck on there, has he been on many dates, is he chatting to many other women, that sort of thing.
I always did. I only ever introduced it as a casual conversation, and answered honestly myself. I think you can judge a lot by someone's reaction what they're all about.
Some people are completely honest and, even if it makes uncomfortable hearing, you can appreciate their candour. Other's will try and 'woo' you with promises and declarations... And out and out lie.
I wouldn't say about casting his net far and wide simply because, however much you don't intend it to, it will probably come across as a little strange. 
But I know what you mean, of course.