My on again off again boyfriend and I got back together 2 months ago and he's already fooling around?

We have been doing this for 8 years now, we have a 6 year old son together. We keep breaking up and getting back together each time we break up it is because of his infidelities. I guess I'm an idiot for expecting him to change but normally we get a 6 month period of him being faithful and trying to be a good dad. Two months ago he came back and I've been supporting him financially because he doesn't work and he doesn't have a dime to his name. Yesterday, I opened my browser on my computer and he was still logged into FB - I saw that he had been emailing several females asking them to get together with him. I viewed the history after this and he had also been in Plenty of Fish communicating with women. I have offered to have an open relationship with him and he refuses to do so because he doesn't want ME seeing other people. I really want my son to have a 2 parent household and am willing to put up with ALOT to achieve that. I really don't know what to do.