The correct ruling in this situation should be that if he has the chips to call the complete bet, then he has to call the complete bet (I've not encountered a card room that differs from this, but if they're re-raising then its definately the case). This is the case because the player has yet to act in this hand. As described below, if the player has already acted then the ruling is different.
FWIW, using your example, with the BB being 200, the amount to call is 200. If he wishes to raise, then the minimum amount is 400. It's not always double the previous amount, though. If you remember that any raise has to be double the amount needed to call then you should be fine making decisions that appear more complicated than they are. Less than this is an under-raise.
It's easier to work in blinds rather than chip values. In an unraised pot it's 1 blind to call, and 2 blinds to raise.
In your example, 200 = 1bb, so the minimum raise is 2bb, 400 chips. He can raise any amount above this, fractions over 1bb are fine.
There are two occasions when it is correct to just call the under-raise, the first being when there's nobody left to act in this or subsequent streets, effectively you're the only one who can win the side pot so you just put in the chips necessary.
The second is when you've been re-raised, or 4/5/etc-bet all-in, and it's not enough extra chips to allow you to re-re-raise (less than half of the minimum bet amount) you can only call. In this situation, calling the complete bet would allow you to re-re-raise, and since you can only call, then you only call the requisite amount. A side pot will be opened on the next street should there be further betting.
