Generally children don't get 7 years for being in a fatally accident involving alcohol. From the sentence, he sounds like an adult who was convicted of vehicular manslaughter. It's not clear where he was arrested, where he's in jail, and what for.
If you would like to marry him, generally the consulate from his country can assist with ID while he's in jail. You need to contact the jail though to find out the procedure their for getting married. His chances of being allowed to live in the US seem difficult. You should consult and immigration lawyer to find out what his chances are. Good luck!
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