I think it is good he was honest. Do not freak out in front of him!. Act like it is all going to be ok and then try to come up with some strategies..
I also suggest you speak with his teacher and have her tell you HONESTLY what is going on with your son and what she would suggest he do to find more friends. Could they tell you what classroom he will be in the fall? Maybe he could invite some of those kids and parents over for a get together at your home or at the park..
Get him involved in different activities, with and without people from school.. Scouts, church group, volunteer group, those fantasy card games. Summer day camps. Music, arts, acting.. whatever he may be interested in. Here in Austin they even have Comedy acting classes for kids..
Do you guys like to ride bikes? See if there are family bike rides available, train for short marathons.. All of these activities will allow you all to make new friends with the same interest. If there is a local festival could you guys volunteer to help this summer?
Being a friend is just like learning any other behavior, there is an art to it.
Get him some books about social situations. I love "How Rude! the teenagers Guide to Good Manners, Proper Behavior. and Not Grossing People Out." by Alex J. Packer, Ph.D. Do not worry, it is age appropriate. It talks about how to be a friend and how to act around other people. It is a funny book too.
I think a lot of kids go through some of this at some point.
This is going to be fine. I really do love that he told you, that is 90% of the whole thing was him voicing his frustration.