family issues. indiscriminate truths

Year or two ago my mother remarried never been sure on the guy. (She's normal) He comes across as ok. But Weve all gotta six sense right. Circumstances happen and recently he met my kids mother for the first time. ( we not together obviously) I made a comment on her tits to gauge his reaction. He had a quick look obviously. Who could resist? Innocent enough. Anyway my son is over this weekend. We are staying at my mothers house while he's here. he's only young and one of the first things the guy asked this morning was his second name (he has his mother's second name) ( he should of known this already. We're not strangers but hey) we all know why he asked! My son didn't say so he asked couple more times. (He thought I was out of ear shot)....a look is innocent enough but for him to pursue it the next morning is enough to pursue my curiosity.......my kids mum is in debt at the minute. I won't think twice about using it and sorting it in exchange for some cheeky seduction & evidence of the misdeeds. Should be enough to precipitate a divorce if my suspisions are right. There is money in my family and leads me to think he's hanging about for the wrong reasons...do I. 1. Pursue it. It Makes my mother a casualty. But know its for her own good. 2. Let nature take its course (not likely anything would happen but if he's interested in others now and is this sloppy it will be destined to happen eventually, better now then down the line). *I'm looking out for someone here give me love lol. 3. Warn him (only thing this will achieve is letting him know I'm onto him). In which case he just be more careful. 4. Open to suggestions?..