He's seen her on the verge of tears several times tonight, but she's managed to control it. Seeing her close again he does what he's wanted to do all evening.

He holds her.

He's not the cop right now, he's the man who loves her and he wants to be there for her.

He'd give anything to make this better, and he will when it comes time to get Boo back. For now he'll give Kate all the comfort and support he can.