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ANSWER: It is possible for him to come to theUS and adjust his status later, but entering the US with a visitor visa, intending to adjust one's status to permanent resident, is a violation of immigration law as visa fraud. The visitor visa requires the person to have non-immigrant intent when entering the US. If he has immigrant intent, then technically he does not qualify.
Many people come, and then later change their minds. Usually, they are safe to apply for adjustment of status after 60 days; I tell my clients to wait at least 90 days if they've changed their minds and want to stay.
Otherwise, he would have to go through the marriage-based immigration process, which can take anywhere from 9 to 12 months. Of course, those are averages - nobody can predict exactly how long it will take due to delays in processing or requests for more evidence, etc; it can take less time, and rarely takes more time. If he has a valid B2 visa, he technically is eligible to travel to the US as a visitor while the marriage-based immigrant visa is processing. But it will be a challenge to convince a border officer that he has the requisite non-immigrant intent with the visitor visa if he is going through the immigrant process.
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