[ Like everything, getting him to offer up his butt will be met with hard resistance at first and the no resistance once he gets over the initial hurdle. Get a bottle of whiskey in him to help lubricate the process and it'll be fine, getting him to do it sober will just be more work than it's really worth. It's not like he's being taken advantage of.
The grip on his shoulder is pretty good as far as communicating goes and he makes an effort to repeat the moves that get a good reaction, zeroing in on tender spots, dragging his teeth with great care over the fabric of the swimsuit before pinching an edge of it between them, tugging and pulling, eventually adding a hand to help get Ianto free. ]