I think it's an excellent product. However, you have to understand what it is and what it isn't.
It doesn't have encounters for all of Undermountain. Instead, it details a portion of the first level of Undermountain, and describes what the other levels might contain.
These detailed areas have remarkably little overlap with those in the original Undermountain set; a few classic areas are updated, but most of the encounters are brand new and placed in undetailed rooms from the original first-level map. (Minor changes are made to the map, but not that many).
The most disappointing thing about the book is that it isn't Book One of Nine. It suffers from not having maps for the deeper levels. The poster map for the Dungeon Level is great; I just would have liked more maps. The original boxed set actually had a similar* amount of rooms detailed, but had maps for the first three levels.
(* I can't remember off hand if my recollection is the original set detailed about 90 areas in total, ~30 on each level, or 90 areas on each level. If it's the former, then this book tracks pretty closely. Otherwise, it's about a third of the content).
The book is greatly helped by it abandoning the strict tactical format used by most of the 4E adventures. Instead, description becomes more key and monster statblocks are only detailed if the monster is not in the Monster Vault. (And even then, some of the monster statblocks are moved to the appendix). It makes it a lot more readable.
I do have to give special mention to the NPCs of the Yawning Portal Inn, which have a lot of nice hooks to use in roleplaying and quest formation.
It doesn't replace the original set, but stands beside it and Undermountain is much, much stronger if you have both products. Either on their own also works as the starting point to an Undermountain campaign, but I do give a slight nod to the original boxed set just for the extra maps and level encounters.
But if you can only get this one? Especially for the needs of your son, he should find it quite entertaining.
Cheers!