90 days after entering via a K-1, he will be out of status. However, once he submits a proper I-485, he will be granted "authorized stay" in the US until the I-485 has been either approved or denied. It is strongly advised to submit Adjustment of Status paperwork within 90 days of entry. "Authorized Stay" is not a legal status, in itself, but it prevents the grantee from accruing unlawful presence.
Some states allow driver's license, etc with proof of a pending I-485.