Is it okay to say:
He will be going to get engaged to his ex-girl friend.
Is there a better of saying this?
First, no. What you have said is not correct.
As for the right way to say it, I'm assuming you mean that they are not engaged yet, but they will be soon. Maybe he's already bought the ring, but hasn't proposed. If that's the case, any of these would work:
However you choose to say it, good luck to the couple. Tell them we wish them the best.