You are either married or you are not married. If you are married to someone else, you cannot marry this intending immigrant.
If you are a US citizen, your marriage to an alien who came into the US with inspection but overstayed his authorized stay may be a basis to waive his overstay. The overstay of the holders on only certain types of visas is waived upon marriage with a US citizen.
If he is in some sort of removal proceedings, his marriage to you now likely will not help him. Similarly, if he has a visa that does not allow adjustment of status while remaining in the US, the marriage will not help.
Your best option may be to review your facts and options with your attorney.
There is no "technically married" -- you are stil married. When you are free to marry your marriage to this man may allow him to gain his permanent residence in the US, regardless of the fact that he overstayed his visa. In most cases, one who entered the US legally and overstayed, no matter how long, and hen married a US citizen may adjust status with no problem.
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