This is a first, after the emotional nuclear war that was his divorce - not a fling, not a temporary tiding-over before the next big event, but an actual, real relationship. He wonders, briefly, if his father would have approved of her. Most likely he would. He would like anyone who could drink him under the table. He abandons that line of thought before it can get dragged into the if-only cycle.
He would like to show her just how happy she's made him, but he's fairly sure he wouldn't be able to finish that promise, and he already knows her opinion on that. So he kisses her, gentle and almost chaste. That will do, for now.