Agree completely with this.
There may be another woman as so many insist, but his desire to keep living together, albeit in separate rooms, suggests there isn't. Contrary to what many on here believe, it is far from true that there is always another woman waiting in the wings.
He may have unrealistic expectations of marriage. Being "in love" in the way you are at the start of a relationship generally only lasts a few years, but you can love each other your whole lives.
He may be depressed. People who are depressed sometimes make life-changing decisions such as leaving their partner, believing that doing so will make them happy again. It generally only works for a short time, if at all.
He may have other mental issues that are leading to this.
Or there may be one of many other reasons.
From some of these, there may be a way back if both parties are willing to work on it, as was the case for @fearfulworrier. From some, there is no way back.
At this stage, you have to assume there is no way back and prepare for the future on that basis. Keep it as amicable as possible for the sake of your son, but be ready for the fact your husband may turn nasty. And remember, this is not your fault.