I wonder if the reason he invited him to his private villa is because he wants to be able to speak frankly and unreservedly.
「自分の別荘に招くのは、腹を割って話したいからだろうか。」=
"I wonder if the reason he invited him to his private villa is because he wants to be able to speak frankly and unreservedly."
腹を割って話す ＝ speak frankly and unreservedly; speak candidly; have a heart-to-heart talk
だろうか ＝ I wonder if ...
アメリカ合衆国 
