is he logged into your apple ID on his phone? have him log out of it. and theres no way to tell what he is able to send/receive as. it could be your number, email address or both. he probably has to restore the phone and log back into his own apple ID. frankly, this was a dumb move on your behalf. he could just turn imessage off in the mean time. that would prevent your messages going to his device as well.