It's not obvious to me his decision will be overturned. The 1st circuit isn't exactly gun friendly, and it's their decision that would be overturned by the Supreme Court.
What happens to him? In all probability nothing, but the process is covered under the US Code Here:
www.law.cornell.edu
TL;dr version. The judge can be censured privately or publicly. They can have cases removed from their docket, and given cr*p cases to adjudicate, or no cases at all. The judicial council can ask that they resign. And finally, they can request that the House of Representatives impeach him, which the House can also do on their own initiative.