Husband's negativity effecting our family: how would you handle it?

I'd use this question as a beginning to your note and add some simple changes you've noticed occur over the course of your relationship. Keep your note positive and attach the heart strings. Make sure he realizes that you and your son need and love him but you need all of him and certainly a better composed, more positive attitude. By the sounds of it he needs some psychological help. Hopefully you can impress this upon him. If none of this works maintain a distance...as you realize you may have a ticking time bomb on your hands that could harm many things. Be vigilant but cautious and let him realize that unless he seeks peace there's just no way you can raise your son in a loving, stable home with him.