You certainly have a lot of good advice. However, I would have liked to have seen the specific stats on your age and his age.
First, if you think he is cheating get a private invesigator or have your close friends keep tab on him.
How's his health? Does he have ED or is he in the early state of dementia? Also, when people have problems such as TIA's or minor strokes there can be personality changes. Sometimes people have TIA's and won't talk about them.
Remember, personality and social issues can be linked to medical issues. Does dementia run in the family? The problems of marrying a much older man is that severe medical issues present themselves earlier than in a marriage where the partners are closer in age.
What is his specific attitude towards you? Does he think that he is superior and that you are trash that can not be trusted? Is he on an worthless ego trip?
How does he treat the children?
Talk to the exwives if you can. I myself wouldn't tolerate the freaky psychological issues. Does he still want this marriage to work? Ask him.
How close your you to getting your degree? Could you make it on your own or live with a friend? I know it would be tuff with the economy, the kids and a divorce.
Could he make child support payments? How's your savings? You have a lot to think about.