Here, "since" probably means since (or after) whatever happened in the previous sentence, though in that case, it would be more normal to say "reunited" than "united" if it was someone he had seen before though. For example, in a news report at 6pm ... "The missing child was found by the searchers at 9 am. He has since been reunited with his parents." ... would mean that the child was returned to his parents at some time between 9 am (the time he was found) and 6pm (the time of the report) - it is much the same as "since 2010" meaning "between 2010 and now".