It sounds like there is a transitional phase ahead for your boyfriend it the US Marshal Service just apprehended him. Depending on the charges, he might be able to have a bond and be released. You could marry then. But, if he is held on a warrant that makes that impossible, then he will be in a federal detention facility until the case is heard. Most likely he will be remanded and unable to qualify for a marriage. Marrying an inmate is not a standard practice. It is very seldom approved and usually for long-term inmates with a perfect behavior record.