well this person certainly need a person to lean on, in this case he is looking a rfuge in you. after all he has done to his wife (there may have be wife faults too). about the 1 night stands. well i would certainly not think he will change in this regard. at the moment cause he has done all this and may be ashamed of himself. he has done this to the end and seen all. may be he has realized. but to find out his true self. if he is given all he wanted care, love, relationship. then he will be able to show his true colors once again.
there is a chance he learn from his mistakes, even a nature can never be changed. he may value you cause of your care and sympathy.
just looked at one line , he is also givng reasons why he done that, like taking drugs.. its really not a valid reason!!
just wait and also watch him reaction towards others, ie, friends, male or female, managers, co workers.
i think if he has changed his behavior , he must have changed towards other people too. not only to people he actually need for some reason.
hope it helps, but dont fall in to this some time, you are right, u dont know about other person he is right or not. so give it time