どんな 顔するだろうな
Does the speaker say "I wonder what kind of face he would make" or "how do you think he'll react?"
Which translation is more accurate? Is he saying this in wonderment or is he looking forward to it, asking someone?
Basically, both of the translations could be correct.
If you have a conversation like
"I'm getting him a present, I wonder what his reaction would be after he sees it"
彼にプレゼント贈るんだ、彼はどんな顔するだろうな
In this situation, the speaker is looking forward to his reaction after he sees the present.
Or it could also be
"I burned his house by accident... I wonder what kind of face he would make..."
彼の家を燃やしちゃった、彼はどんな顔するだろうな
In this situation, the speaker is saying this in wonderment, also the speaker has an expectation of his reaction already.
どんな 顔すると思う?
instead.