Let him do it. As this is Ravenloft, this will make him a Domain Lord (read: NPC) in no time.
Beyond that, he will attrackt a lot of "fans" with that, which will be a problem.
And of course, lots of shadows participating in a fight will mean that the fights will be very easy, resulting in next to no XP for the party (the players will hate him for that).
But there's another thing that strikes me as weird: You said this guy read the Monster Manual - during the session - and wants his character to use the knowledge he as a player gained through this reading. Doesn't that sound odd to anyone here?
His character isn't supposed to know this stuff. Let him make a Knowledge(religion) check to know about shadows. Since the character hasn't even seen the things (or so it sounds), the DC will be higher than usual. And then, he'll learn the part about shadows created in this way are under the original shadow's control last, he'll learn everything else first. This will mean that he would have to make a hell of a roll (DC 13 base, plus 5 for every additional bit of info. So first he hears about HD and monster type and subtypes. Then ability scores. Saves. Skill bonuses. Strength Damage. Creates other shadows. These are under the original shadow's control. This should be beyond 40).
When his character rolls and doesn't get the information, tell him that if he tries to metagame to get the character to learn the information, you metagame to let his enemies know about his plans and stop him
![Devious :] :]](http://www.enworld.org/forum/images/smilies/devious.png)