How close are you to him? (Are you friends, lovers or just someone you see occasionally)
Are you still in love with him or wish to get back with him?
How long have you been apart?
Do you still do things together? (Such as hang out or talk often)
It is very kind of you to try and help him, the only problem is if he has this cynical out look on women and you are not as close as you once were he will likely see you as working to get back into his life. While that may not be true the reality is that is the way he likely will see it...........thus reinforcing the idea that women are sneaky and conniving.
The fact is people come with baggage......and sometimes they carry this around all their lives until they meet someone they can fully trust and learn to let go of the past. There is no way to open his eyes other then proving to him he is wrong. Since you are no longer in a relationship with him there is no way to prove to him this is the wrong out look. Sadly some women are sneaky and conniving.....just like some men are.
The good news is there are things you can do to show him that you are not like that and thus not all women are either. The first step is to simply keep being his friend. Since you are not in a relationship with him this helps show him that you have nothing to gain from him yet want to be his friend regardless. Sometimes men can be like children......as much as I hate to say that But the fact is we can be! Thus some of the same tools you use on children often work well with guys too! When talking to him if he starts in on how women are like this.........ask him who does that he personally knows. (If he pops off his mother, explain that there are always two sides to every story and she was not like this with him so she does not count). If he can name someone try to get him to talk about why the relationship went bad. If he can not name someone or becomes frustrated simply tell him that while some women can be flaky so can some men.....but he is proof that all men are not like this.
Redirect.....Redirect.......Redirect............anytime he starts in complaining about women turn it around into something positive. Such as:
The key is to not disagree with him........but to show him that all these things are just part of life and love. And to find the positive in it. Yes we all are going to have someone who betrays us or hurts us but if we all gave up no one would ever find lasting love. I never say true love because the fact is that is a myth! Love is not about being slapped in the face with it and walking off into the sunset happily ever after. Real lasting love is working with each other and being understanding that everyone makes mistakes.......that is what makes it lasting.
No one is perfect and that is what he must come to accept and deal with. Yes women will slip up and hurt the man the same as men will slip up and hurt his women. The key is accepting this and learning to forgive each other and move on. If we always dwell on what someone else has done we can never learn to get past it.
I know you want to help him........and that is great. Though the reality is he can not change until he is ready to do so. No matter how many times you show him women are not all crazy the fact is he will have to learn this on his own to fully believe it. The good news is with time he will come though this, just be his friend until he does.