If you share an Apple ID, I don't think there is a way to prevent this from happening. The only thing I can think of, besides getting separate ID's (which is not hard and has its benefits) is to check his settings. Under messages, it will say send/receive as. One is your number. The other is your Apple ID. Disable send/receive for the Apple id on one or both devices. If that doesn't help, create a new Apple ID for one of you. That is not a carrier issue. It is an apple issue.
@Mahm: it is posible but you need to know what you're doing. The only benefit to sharing an account is being able to shaw music. For some people it's not worth the trouble 4951