If you have any experience with them, consider putting some of the things that make intriguing storylines from video games INTO your adventure to catch his interest. I can't say for certian that he'll go for it, depending on which games he plays, but there are plenty of 'gamer' ways to draw a person's attention.
When a gamer is looking for a cue as to what they need to investigate next, an NPC will, nine out of ten times, be able to guide them in the right direction. Maybe in your example of certain townsfolk keeping monsters as pets, you could have a local policeman point out "hey, you notice how so many people have been adopting those monsters around town? Well, truth be told..." If they encounter this situation as an obstacle, rather than a riddle 'on the side', they're more inclined to pay attention to it, and the best way to do that is to feed them the riddle through the mouth of a trustworthy NPC.
So maybe they don't care if the town guard is doing their job, and just want to get to the next 'dungeon' that they can explore. This isn't a problem either, necessarily. Assuming they can follow simple directions, and at least ONE of your players are tryign to figure things out, you can feed them simple instructions on how to proceed. Maybe it will ruin the riddle for certain players, but if they're not going to figure it out on their own, trying to drag them thorugh it is only going to frustrate them (and you) the longer you let it drag on. Nothing is more frustrating to a gamer than being stuck in Town X talking to every NPC until the right one feeds you the right dialogue to move the plot along.
A Memorable Character
If you want to catch a player's attention REALLY well though, make the person dispensing this information as memorable as possible. Town Sheriff may be an important job, but is just about as important to a PC as the village idiot. Instead, make it a half-dragon Town Sheriff named Goliath who picks his teeth with his sword and chews tobacco then spits it out in a plume of fire. Or give him a moment where he busts apart a theft before addressing the PCs to assert his awesomeness. Or just give him a distnict scar across his cheek that he mutters 'bloody rusty razors' about when asked. If they remember this character, they will care about what they have to say.
Keeping a consistent thematic progression to your story also helps keep the player's attention. Fighting monsters without a care in one dungeon, only to be solving a riddle about why they're being kept as pets aftewards, may confuse a player, because they're in the mindset that they don't need to care about monsters at all. Instead, introduce one of the monsters early on as being sympathetic, maybe even after they've nearly killed one, and paint them in a light so that it progresses naturally that such a thing might happen, or that such a thing is incredibly obviously wrong and off-putting.
Finally, if you want a player to sympathize with a monster, you need to put a lot of effort into that, even if they aren't in 'a gamer mindset', because let's face it, we have all played a tabletop game where we didn't even think about the fact that we've slaughtered hundreds of nameless magical beasts for no real reason other than 'they were in the way and gave us EXP'. It's entirely too easy to not care about these encounters. Instead, make intelligent monsters chatter to one another and form tactics. Make wild monsters roll around and knead at the ground to make soft beds of dirt before the PCs show up and they attack. Have a skeleton tip their entire head to the party before engaging in honorable combat.
The more memorable a character or situation, the more someone is going to care about them later on, regardless of their past gaming experience.