Ronan isn't used enough to the sensation to hold on, to keep his typical aloofness or stoicism. Besides, he's already more than consented to whatever else is going to happen in this room. This is unexpected but incredible, and the sounds he makes indicate that he feels like that. 

"Fuck, Frank, please," he finally says, all but writhing under the older man's tongue. He keeps his promise not to kick him, but it's impossible to stay still. He's actually pretty sure he can get off like this, without attention directly on his cock, and that's a realisation he's never had before. It's kind of embarrassing, actually, so maybe that please is asking for something other than continuation, but he doesn't have the coherence to make a real sentence about it.