We've recently had this issue, and we paper trained as well. From research and talking to our usual dog trainer it's one of the possible bad habits in the dogs 'terrible twos' stage of life (think of how badly behaved a toddler can become in this phase of their lives as well). Some dogs start to bark when they didn't, some go toilet in the house, some go back to chewing everything... Some dogs do nothing bad at all as not all dogs go through this phase, and they do grow out of it.
However in the meantime your dog will need a bit of tough love. Our 8 month pup spends his nights in a smaller pen than he was used to (open plan house so we don't want him having the run of the place overnight). He used to get the whole kitchen but now he doesn't until he re-learns his house training. He's left with a few quiet toys, and his food/water bowls. We started with a pen, with his little crate in, just big enough for him to walk around it (or sleep on the cold tiles if he gets hot) and now we're at a larger pen over double the size of the original one, which is now about 1/3 of our kitchen. He's still dry overnight and in the day with this method. The pen will keep getting larger the more dry days/nights he has until he's back to the full space he was in.
Leaving his food and water bowls in there, even empty, we found helped - dogs have the mentality of not going to the toilet where they sleep (crate) and where they eat (food bowls). We used to take his food bowl away after breakfast / dinner.
However in the day he's only in a pen if we're out, otherwise he can run around downstairs as much as he likes as we're there for him to ask to be let out. We know, just like you, that he is perfectly capable of holding for 9 hours (though he doesn't usually have to on a regular basis), but it is a bit of a spiteful habit when a dog is in this phase.
Your dog needs structure - as when I asked How to stop overnight toileting on here myself, I got some great advice which has helped. If you're able to regularly take him out when you're in, he'll get more used to empting his bladder when he gets the chance to - plus he'll get more bored of where he is going, so less excitement = more likely to pee.
We took Liams (above question) advice of a regular evening pre-bed walk. This walk is the exact same route every night - he knows this walk, and what it means, and consistently he'll at least pee on this walk, even if he doesn't poo. 8pm every night he get's agitated and asks to go on his walk. This in combination with regular meal times and not over treating or feeding the dog can help with less number 2's (and is just plain better for your dog!).
We've also found going back to giving him a treat after toileting outside has re-encouraged his want to go out for a toilet. We also recently have found adding in another walk to the day is helping.
Plus you need to learn your dogs quirks for asking to go out - our Pup doesn't always go to the back door and whine, sometimes he sits as high up on the stairs as he can and just stairs at us.
