Why would a guy decide to keep in touch with some ex girlfriends and not others?

ok...personally...i am friends with all but one of my exs...the ones I am friends with ..it is because they have all grown to be some of my best friends...and give me the best advice in time of need when it comes to women now...and the one that I am not friends with...is because she just couldn't accept that fact...she couldn't have anything to do with me unless it was in a relationship.
regardless though...
all of my friendships with my exs started out exactly how you have described...it wasn't something that happened right away at all...some of them took a couple months...some a year...one even took four years before we were really good friends.
just give it a little bit of time...he is clearly capable of being friends with ex girlfriends...so it shouldnt be a problem for him to be friends with you. it is probably a process he goes through...of breaking up and then trying to get over you. I have done this on a couple occasions myself. he just needs some time. I'm sure he will come around. and when he does welcome him as a friend if you really want him as one and can accept that he is probably going to be dating people and getting new gfs. this is really important if you want your friendship to work out.

hope this was helpful

Brian from www.ideabombs.com link