Your son sounds like a trooper!
The conference things is wise only in that your son will see you and your husband explaining the "new world order." Keep it very simple: Daddy is not going to be living with us but we both love you very much. Daddy will see you at such and such times and we both love you very much. Daddy leaving is NOT YOUR FAULT and we love you very much." Ask him if he has questions, answer them simply (I don't know is actually a good answer if you really don't know), end the meeting and move on.
Your son will pick up his "how to react" cues from you. Make sure you handle this with aplomb and dignity and so will he.
As the child of multiple divorces, here's a few longer term thoughts...
1. Never trash talk your husband around your children. That's more unsettling than him leaving.
2. Never promise something you can't deliver. "I'm sure Daddy will..." sets everybody up for anger and resentment, if Daddy can't do what you promised (and vice versa).
3. If at all possible, avoid the revolving door. If your husband is leaving, your relationship is OVER. If there's any chance of saving it, please reconsider his moving out. If he leaves and returns, make sure it's for good. It is so unfair to teach your son that problems in relationships are solved by leaving and coming back. Can you think of any girlfriend of your son's (when he gets old enough) who would put up with that? What would you think of such a girl?
4. Don't make excuses for your husband. If he breaks visitation or promises, don't "explain it away." That's your husband's job. Let him suffer the consequences of his actions. It's heartbreaking but true. Be honest in a non-judgmental way. For example, "But, mommy, this is the 3rd weekend in a row Daddy was too busy for us. Doesn't he love us anymore?" "Yes, sweetie, he does. I have no idea why he acts this way. Why don't you ask him when you see him next?" My husband worked insane hours when my kids were little, which only changed recently. Now, as teens, my sons are skeptical that my husband will actually join us for family activities. My husband once chided me for not "covering for him." I gave my beloved husband a big hug and told him that HE acted this way, now HE can fix it (plus I warned him of my intentions when the kids were born). My husband learned a valuable lesson almost too late.
Good luck!