I definitely wouldn't tell him you found it! Let it be a complete surprise. You got a quick glance at the ring so you know he's planning it but you don't know when or where. When the time comes you'd probably guess what was up anyway so it's not like knowing he's planning it will give anything away and you'll still be surprised. Once you get a good look at the ring you might realise you like it more than you think. Heck, when he proposes he might even say something like "we can change it if you don't like it" (I wouldn't take him up on that unless it was truly godawful, though!).