Why doesn't he want to get married right away?
I agree that it's a huge risk. It's one I wouldn't want to take.
That being said, my sister got married early for tax reasons and her husband had trouble "lying" to friends and family so ended up telling everyone. He had this idea of a perfect wedding, perfect whatever, and that being The Real Deal. Doing it early for practical reasons played on his mind a lot even though it's quite common and perfectly valid. Why does your partner not want to get married now and mitigate your visa risk, separation, and additional cost (flights, a place for you stay, etc) if you having to go back to the US for a couple / few months?