Hi Fisch,

I would suugest that your boyfriend gets AC to tag your luggage all the way through to BRU.
Upon arrival at LHR (Terminal 3) follow signs to the Flight Connections Centre (5 minute bus ride)Once through the security search at the Centre on the left hand side there are a large number of BA transit desk where he can get his boarding pass for LHR-BRU ( show then your BA ticket with the baggage tag ).Then it is down one floor with the escalators to Terminal 1 international departures lounge. Then relax and watch the screens fro the BRU flight.
There should not be any problems with the baggage transfer. ( I have done it between BA and Virgin).

Hope this helps

Regards
TBS