The 2 week program/stay with dad sounds like a good experience for your son, plus it give bonding time with step-sister and best friend.
I am uneasy about that rest of that month's stay where 3 kids are at home to do whatever while dad sleeps. The one child is 3... are the 7 year olds suppose to discipline and watch that child, bad idea. I would suggest to the dad that during the time he is sleeping a babysitter is found to watch your son and along the other 2 kids. It is very unsafe to leave kids that age unattended too, even if an adult is sleeping in the next room... kids that age can get outside (even if asked not to go outside), what if they need something. That is not a cool situation.
I have to admit that there has been times I have fallen asleep while reading to my daughter or while playing on the floor with her. I never stay sleep more then a few minutes and then wake up and check on my daughter... then make a bee line for some caffeine! Guys usually don't respond that way or quickly, once they are out they rarely hear that child crying, yelling or so on.
So as I said above see if the dad would be willing to get a babysitter for those hours of rest. Maybe cheap in a bit for what it would cost to watch one kid (your son, $15 a day) and have the dad pay the rest? If he says no then say you are not comfortable with leaving your child unattended (even with him asleep in the house, because issues/problems happen quickly and need that adult supervision) so if the son is to stay over for that month he has to have an awake adult to watch him or he will not be staying for the full month.