Should we try to work it out?

My husband and I have been going through a very rough time. I kicked him out of the house about 7 months ago but had to let him back because I knew I wasn't ready for a divorce. We've mostly ignored each other since then. I recently found out he cheated on me. While there is certainly no excuse for that, I can identify with his feeling that I wasn't being much of a partner, so when he found someone interested in him...
My first instinct is of course to drop him, but we also have a five year old son. While I don't believe in staying together for the kid (mom's got to be happy too), do I at least owe it to my son to try and suck it up and work this out?
Should I call a divorce lawyer or a marriage counselor?
We are able to sit down and talk about it, but that really doesn't solve anything. I just don't know how I can trust him again. If our relationship was better, would he be trustworthy?