Sarah,
ASK HIM!!! Seriously, if he's not doing something anymore there's probably a reason, and none of us are going to know that, only him! So when you next feel ok bringing it up, just ask, "You used to luv going down on me, but now it's like you don't. What's wrong? Maybe we can fix it?"
That just sounds the most logical thing to do in my opinion. There could be any number of reasons - maybe it's just lost the novelty appeal and now it's boring or something. Like I said, I dunno, and nobody else but him probably knows.

Good luck, and let us know what he says!
CasperTG

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You know, Blackadder, for me socks are like sex. Tons of it about and I never seem to get any.

My Photos:
https://imgur.com/a/80hiRdm