I am normally one of those that say be honest. And normally would say try to make amends. But I am going to go against this at this point. What is the big deal about not telling her? It is just until in December - less than a year. Hardly anything to wait.
I am also guessing you are planning on not getting married until after he finishes school any way. If you really do not plan on having her a part of your life why would you care either way if she knows or doesn't know?
I am guessing you probably want her out of your life now - maybe that is why you want to tell her. So you can be done with her now. I can certainly understand that. Personally putting up with this craziness for another 7 months at most seems a small price to pay (pun meant) to come out of this debt free tuition-wise. Even if you have the money to help. You can certainly use that money to help start your new life together.
One question -- do you feel confident he is going to break it with his mom? That would be the only worry I would have. For 7 months and to be able to start a marriage with little or no debt is really worth it - save any money you planned to use to help him pay for a down payment on a house. And then you can thank your MIL for the "wedding gift".