S.,
I'm sorry to hear about what you are facing. It is never easy. When my now ex and I separated I simply told our children that Mom and Dad weren't going to live together anymore, but we both still love them very much. I reasured my daughter that it wasn't anything that she did, and just made sure she knew that I love her.
My situtation isn't quite the same as yours my kids were 5, 3, and 10 months when we separated. They didn't really have much of a relationship with their father, and they seem to have adjusted well. My ex, however, told my daughter that Dad made some bad choices (like cheating on me repeatedly, sorry I had to throw that in there) and Mommy didn't love him anymore. I felt like smacking him. Please try not to dump your emotional baggage on the kid. Find a support network and vent to them, preferably when your son isn't around.
I do my best not to bad mouth their Dad when my kids are within earshot. Even when I'm on the phone with other people, I've found I have to watch what I say when my oldest daughter is home from school. I have also found that when he plans to do something with them it is easier not to tell them about it until he actually shows up, rather than face their disappointed looks when I have to give them his latest lame excuse why he can't do it afterall.
If you feel that you or your son need couseling I would do it. It is nice to have help through this transition in your lives. As far as discipline goes, I would do my best to maintain the current rules you have already established. This probably isn't the time to introduce new rules, but maintaining the old rules may give him some stability in a world that is otherwise being turned upside down.
I hope at least some of that is helpful. Make sure you have a support network in place find good friends, a counselor, family, church, support groups, etc to help you. Being a single Mom isn't easy, and it helps when you have people to turn to for support. I wish you the best as you and your son make this difficult transition.