Is it okay to say:
He will be going to get engaged to his ex-girl friend.
Is there a better of saying this?
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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:
He is getting engaged to his ex-girlfriend.
Soon, he will be engaged to his ex-girlfriend.
He and his ex-girlfriend are soon to be engaged.
He and his ex-girlfriend will be engaged soon.
He and his ex-girlfriend will be getting engaged.
He and his ex-girlfriend are getting engaged.
That last one may imply the engagement is more imminent than the previous example.
However you choose to say it, good luck to the couple. Tell them we wish them the best.