Reddit mentions of The Princeton Field Guide to Dinosaurs: Second Edition (Princeton Field Guides (110))

Depending on his interests, here are a few options:

• Model dinosaurs: There are several companies that sell accurate scale models of dinosaurs and other prehistoric life; Safari LTD and Collecta are two of my favorites. Beasts of the Mesozoic are some slightly higher-end poseable raptor models.
  
• If you think he'd be interested in a print of professionally-made dinosaur art, a couple of paleoartists whose sites I could find are Emily Willoughby and Mark Witton.
  
• If he might be into stickers, shirts, or other assorted dinosaur-themed merchandise such as mugs or phone cases: Permia sells shirts, stickers, and cards. Some paleoartists have Redbubble pages or similar. A few I recommend looking at: [1] [2] [3] [4] [5] [6] [7]
  
• If you think he'd be interested in scientifically accurate plushies, Palaeoplushies might be worth looking at.
  
• Books: There are a wide variety of paleontology-related books. Depending on what he's read, and what he's interested in, here are a few possibilities:
  
  • Books about prehistoric life (mostly dinosaurs) in general:
    
    • Pterosaurs by Mark Witton
      
    • A Field Guide to Mesozoic Birds and Other Winged Dinosaurs by Matt Martyniuk
      
    • The Rise and Fall of Dinosaurs by Steve Brusatte
      
    • The Princeton Field Guide to Dinosaurs by Gregory Paul
      
    • Dinosaurs: How they Lived and Evolved by Darren Naish and Paul Barrett
      
  • Books about paleoart:
    
    • Extreme Dinosaurs 2: The Projects by Luis Rey
      
    • The Palaeoartist's Handbook by Mark Witton
      
    • Recreating an Age of Reptiles by Mark Witton
      
    • All Yesterdays by John Conway, C. M. Kosemen, and Darren Naish

I have this. I like it, and it is fun to thumb through it. But as another user said, shit gets discovered pretty quickly though.

Gregory Paul's Field Guide to Dinosaurs is pretty big and scientifically accurate for the most part. Some of the length is just brief descriptions of dinosaurs, however, and I wish it went into more depth.

I'm a big fan of Mark Witton and hope to order Recreating an Age of Reptiles soon. For some reason Amazon says it isn't out yet, but it was published a while ago. I find that Witton always does very meticulous research and does a great job of acknowledging and explaining disagreements. While not about dinosaurs, his book Pterosaurs is my favorite paleo book of all time. I love the artwork and it's clear that he has a real passion for these animals. He really goes into detail about different groups of pterosaurs, too. I felt like I learned a lot about pterosaurs reading the book and it really made me excited to read anything else he has written. His blog is also fantastic if you are looking for something to read!

As far as up-to-date, The Rise and Fall of the Dinosaurs was just released and seems really interesting. It's has less of a textbook/encyclopedia feel than the other books I've recommended here and it seems like it might follow more of a narrative compared to the others which jump between types of dinosaurs. I haven't read it but I've heard good things and am excited to read my copy soon.