Unless he has money that is solely his, he should not be able to purchase a house at this time. Not with joint funds anyways.
Divorces take a while, those houses may not be for sale by the time he has access to the funds to buy. You need to file immediately, here in MO they issue an immediate judgement to what you can and cannot do w/joint funds.
And if all else fails, don't tell him in advance, and if he buys the house, move. Be careful since you have shown your cards so to speak. Get your importants out of your house for a while. Ugh, I am so sorry.
Tight hugs to you and your children!!!