I think you can try to I-485 to join you, he is dependent, possible to be accepted it. If it is rejected again, record all response from CIS, and let him out of status, upto he can file I-485.
In future, if he had a RFE, you would show all evidences (response of CIS), the RFE can easily passed, since you are legally stay, he (your dependent) should be ok.