Hi neptun1982, I have seen a story like this in the news, although the amount paid in error was in thousands not a few hundred. The person concerned spent the money, was prosecuted and went to prison!
I am sure this would not be the case, but the fact is that your boyfriend realises that the money does not belong to him and legally he has to return it. I am sure the bank will find out and want their money back, I also believe that they have a right to ask for interest as well if you keep it for any length of time.
The best thing to do is to contact them and let them know. You could try charging them for your time spent in having to deal with this and negotiate a small reduction on the amount you pay back! I would as this has put you to some trouble!
Cheers Sue.