A case similar to [1] this one -- which sheds light on why the husband is able to do this.

From [2] Kyodo News (thanks to Twostellas):

PUTRAJAYA — A Malaysian Hindu woman began her final bid Monday to get Malaysia's top court to prevent her estranged husband from converting their son to Islam.

The story goes on to say that this case is being closely watched, as it deals with the increasingly vexed question of the overlap between Sharia and civil law.