I agree that something formal needs to be put in place. Where I live, there is a mandatory 6-month separation necessary before a divorce (exceptions can be made in extreme circumstances). But this ties a nice loop around that - a legal separation definitely establishes that it's happened. It also protects you in many ways moving forward - from accumulating debts to the business venture that your husband is embarking upon. You should ask your lawyer about different possibilities before deciding since the business has both liabilities and benefits to you, depending on how it goes.
There should definitely be a child agreement put into place. Try not to get too hung up on the specifics, but look at the bigger picture. You can hammer out working details later, as time goes on and the situation changes.
I know you said that you wanted to wait until after your son graduated from H.S. Only you know what's best for your family and their personalities and your situation. However, consider that having your child leave home and then having his home environment change while he can't see it may make him feel as though his home base no longer exists. It truly might be easier for him to go through this now vs. later. This is especially true if your husband remaining in the house is more stressful to your child than him leaving. It could actually be a relief. We parents often assume that we know how our kids are going to feel/react to things, and I'm often dead wrong. Find a way to gauge your children's feelings without it being too intrusive or scary for them. Just start a conversation about how their dad makes them feel, or how they feel when he's not home vs. when he is, etc. You may be very surprised at what you hear.
Kel