there is more than one possibility. Sometimes the
construction is with ir + a + infinitive sometimes with
the simple future I. For expressing a presumption only
the simple future I can be used.
me he will come. Will you
buy it ?
He will know what he is doing. Are you going
to keep quiet now
You will have to tell him. I think that
he is not going to do it.
You will regret this,
I swear. What I do
I know he will be sick.
Ask him whether he will do it
or not. Probably
he does not have any money left.