It is possible to come as a visitor, and then adjust status after marrying a US citizen. But usually only in circumstances where the person came with the intent to visit only, and then later met and married a US citizen and decided to stay. And usually, they are only safe from questioning of intent (discussed below).
But the problem in this case is that if he is coming to the US for the purpose of (with the intent to) getting married and staying in the US, then he should not use the visitor visa, as that would be a violation of immigration laws. It could lead to big problems.
The reason this is the case is because the US immigration authorities do not want people circumventing the appropriate channels (which take anywhere from 9 months to 1 year (CR-1 spousal visa) or 6 to 9 months (K-1 fiance visa)) so that they can get to the US and get married and stay without waiting these processing times. In other words, there is an appropriate process for this (either K-1 fiance or CR-1 marriage based visas), so using the inappropriate methods to get around long waiting times is a violation of immigration laws.
The right way to do this would be to petition for his K-1 fiance visa, and then for him to come in about 6 to 9 months, get married within 90 days, and then apply for adjustment of status to get his green card to live and work in the US. K-1 visa information: http://travel.state.gov/visa/immigrants/types/types_2994.html.
If he enters as a visitor, he must get married and submit the application for adjustment of status before his 90 days expires. Otherwise, he will be asked to leave. The problem with this is that if a person comes as a visitor and within the first 90 days, gets married, then when he submits the application for adjustment of status, it could very well raise questions as to his intent when he entered the US. He would have to show that he did not intend to get married and apply for adjustment of status when he entered the US as a visitor. This is not very easy to do.
So if he does what you are planning to do, then he needs to be very prepared to explain that he did not have the intent to get married when he entered with the visitor status. That is not going to be easy to do, so you both will need to get an attorney to help, if that's how you go. But again, this is an illegal process and it is still possible to get a denial. If that's how you go, the longer you wait to get married and submit the documentation and applications and petition, the better.
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