After the short sale, they can sue for what is called a deficiency judgment for the outstanding balance of the mortgage. They don't just automatically get it, though, and some states prohibit the practice. Connecticut is not one of those states, though, and they can pursue him. Once they get it, it is a judgment like any other. Unlike the original mortgage, it is not secured, although it can be used, after domesticating it in Washington state, to file a lien against his property there, which is a secured interest.
Or they can attempt to garnish other sources of income or assets he has, if they feel that's worth their trouble.