Far from 'letting' him top, Jack simply loved to bottom. Sure, he tended to be dominant in the bedroom a lot of the time, simply through sheer force of personality and amount of experience, but he did love to get fucked and, equally, he loved to be dominated himself, if someone had a strong enough personality to manage it.
Or, to put it simply, he loved pretty much anything in the bedroom (or anywhere else that seemed suitable at the time). Different things for different partners and occasions, and he'd had a good fucking last night so he had no hesitation at all to conceding to Kirk's prompting with a slow smirk that promised all sorts of wonderful things.
"Well, I guess if you want to be impressed..." he drawled, because with all that talk about keeping up there was really only one way this was going to go, "I guess I'd better find the lube and start prepping you, hadn't I!"
Not moving just yet, though, because his cock was still being squeezed by very talented fingers and that felt too good to just slip away from.